['Biological!', 'systems.', 'exhibiting', 'transduction', 'crosstalk', 'However,', 'both', 'the', 'cellular', 'levels', 'of', 'pro-', 'filin,', 'as', 'well', 'as', 'its:', 'affinity', 'for', 'actin,', 'appear', 'too', 'low', 'for', 'it', 'to', 'play', 'this', 'role', 'in', 'vivo”.', 'The', 'role:', 'of:', 'profilin', 'in:', 'vivo:', 'is', 'more', 'likely', 'to:be', 'related', 'to', 'its', 'capacity', 'to-catalyse', 'the', 'exchange', 'of', 'adenine', 'nu-', 'cleotides', 'on:', 'monomeric', 'actin”.', 'During', 'actin', 'filament', 'formation,', 'ATP', 'bound', 'to', 'G-actin', 'is', 'hydrolysed', 'to', 'ADP,', 'which', 'remains', 'tightly', 'bound', 'to', 'F-actin”:', 'During', 'filament', 'remodeling,', 'ADP-actin:', 'monomers', 'are', 'released:', 'However,', 'while', 'ADP-actin', 'monomers', 'can', 'polymerize,', 'ATP-actin', 'monomers', '.do.so', 'more', 'rapidly', 'and', 'form', 'more', 'stable', 'filaments', 'than', 'their', 'dinucleotide-bound', 'counterparts?.', 'Since', 'the', 'exchange', 'of', 'ADP', 'for', "ATP'on", 'G-actin', 'is', 'rate', 'limiting,', 'profilin:', 'has', 'the', 'potential', 'to', 'promote', 'actin:', 'polymer-', 'ization', 'and', 'filament’', 'stabilization', 'by', 'catalysing', 'actin', 'nucleotide', 'exchange”.', 'This', 'has', 'been', 'shown', 'in', 'cells', 'that', 'stably:', 'overexpress', 'profilin,', 'where', 'actin', 'fila-', "ment'stability,", 'measured', 'by', 'the', 'fluores-', 'cence', 'decay', 'of', 'microinjected', 'caged', 'resorufin-actin,', 'is', 'directly.', 'proportional', 'to', 'profilin', 'concentration', '(T:', 'Finkel', 'et', 'al.', 'submitted).', 'However,', 'the', 'observation', 'that', 'microinjected', 'profilin:', 'decreases', 'cellular', 'F-actin', 'and', 'prevents', 'the', 'ex-', 'tension', 'of', 'lamellipodia.', 'indicates.', 'that', 'at', 'high', 'concentrations,', 'profilin:', 'can', 'sequester', 'actin', 'monomers', 'in.', 'vivo’,', 'In', 'contrast,', 'the', 'microinjection', 'of', 'high-', 'affinity', 'profilin-actin.', 'complexes', 'in+', 'crease', 'F-actin', 'and', 'enhances', 'membrane', 'ruffling’,', 'consistent', 'with', 'the', 'effect.', 'of', 'profilin', 'on', 'actin', 'nucleotide', 'exchange.', 'Profilin.', 'binds', 'to:', 'polyphosphoinositides.', 'Pro-', 'filin', 'binds', 'tightly', 'and', 'specifically:', 'to', 'four', 'or', 'five', 'molecules.', 'of', 'phosphatidyl-', 'inositol', '4,5-bisphosphate', '[Ptdins(4,5)P,]', 'in', 'vitro,', 'and:', 'this', 'results', 'ina', 'dissoci-', 'ation', 'of', 'the', 'profilin-actin', 'complex’.', 'Concomitantly,', 'profilin', 'masks', 'Ptdins-', '(4,5)P,', 'from:', 'the', 'hydrolytic:', 'action', 'of:', 'phospholipase', 'C-y,', '(PLC-y,)*.', 'Thus,', 'pro-', 'filin', 'provides', 'a', 'link', 'between', 'a', 'major', 'signal', 'transduction', 'pathway,', 'the', 'phos-', 'phatidylinositol', '(Ptdins):', 'cycle,', 'and', 'the', 'actin', 'cytoskeleton', '(Fig.', '3).', 'Ultrastruc-', 'tural', 'localization:studies', 'demonstrate.a', 'significant', 'fraction', 'of', 'profilin', 'in', 'close', 'association', 'with', 'both', 'the', 'internal', 'leaflet', 'of', 'the', 'plasma', 'membrane', 'and', 'external', 'surfaces', 'of', 'intracellular', 'mem-', 'branes', 'in', 'human', 'leukocytes', 'and', 'platelets”.', 'This', 'association', 'with', 'the', 'membrane', 'is', 'likely', 'to', 'reflect', 'its', 'direct', 'binding', 'to', 'phosphatidylinositol', '4-phos-', 'phate', '[PtdIns4?]', 'and', 'Ptdlns(4,5)P,.', 'Stimulation:', 'of', 'platelets', 'with', 'thrombin', 'results', 'in', 'rapid', 'PtdIns', 'turnover,', 'the', 'cycling', 'of', 'profilin', 'between', 'the', 'cytosol', '442', 'and', 'the', 'plasma', 'membrane,', 'changes', 'in', 'the', 'levels', 'of', 'the', 'high-affinity.', 'profilactin:', 'complex.', 'and', 'in', 'the', 'polymerization:', 'of', 'actin”.', 'However,', 'it', 'is', 'important', 'to', 'rec-', 'ognize', 'that.', 'a.', 'number', 'of', 'other', 'actin-', 'binding', 'proteins,', 'including', 'gelsolin,', 'gCap39,', 'severin', 'and’', 'protein', '4.1', 'also.', 'bind', 'to', 'Ptdins(4,5)P,', '(Ref.', '28),', 'and’', 'any', 'scheme', 'devised’', 'to:', 'explain', 'the', 'con-', 'nection', 'of', 'the', 'PtdIns', 'cycle:', 'to:', 'actin', 'remodeling', 'would', 'also', 'have', 'to', 'account', 'for', 'the', 'actions', 'of', 'these', 'other', 'actin-', 'binding', 'proteins.', 'The', 'receptor', 'tyrosine', 'kinase-profilin', 'con-', 'nection.', 'As', 'discussed', 'above,', 'PtdIns(4,5)P,', 'that', 'is', 'bound', 'to', 'profilin', 'is', 'not', 'hydro:', 'lysedi', 'by', 'PLC-y,*', '(Fig.', '3).', 'However,', 'when', 'PLC-y,', 'is', 'phosphorylated', 'by', 'the', 'action', 'of', 'receptor', 'tyrosine', 'kinases', '(RTKs),', 'such', 'as', 'the', 'receptors', 'for', 'epi-', 'dermal!', 'growth', 'factor', '(EGF)', 'or', 'platelet', 'derived:', 'growth', 'factor', '(PDGF),', 'it', 'be-', 'comes', 'capable', 'of', 'hydrolysing:', 'profilin-', 'bound', 'Ptdins(4,5)P,', 'to', 'generate', 'inositol', 'trisphosphate', '(Ins(1,4,5)P,}', 'and', 'diacyl-', 'glycerol”.', 'This', 'suggests', "that'", 'profilin', 'is', 'now', 'free', 'to', 'interact', 'with', 'actin', 'and', 'sta-', 'bilize', 'filaments', '(Fig.', '3):', 'It', 'is', 'well', 'known:', 'that', 'growth', 'factors', 'such', 'as', 'EGF', 'andi', 'PDGF', 'influence', 'the', 'actin', 'cytoskeleton:', 'and', 'induce', 'cells', 'to', 'change', 'shape,', 'ruffle', 'their', 'membranes,', 'migrate', 'and', 'divide.', 'However,', 'the', 'mechanism', 'by', 'which', 'RTKs', 'influence', 'the', 'organization', 'of', 'the', 'actin.', 'cytoskeleton', 'is', 'unknown.', 'Also', 'obscure', 'is', 'the', 'mechanism', 'by', 'which', 're-', 'ceptor', 'tyrosine', 'kinases', 'activate', 'inositol', '1-phosphate', '(Ins1P)', 'turnover,', 'since', 'the', 'phosphorylation', 'of', 'PLC-7,', 'does', 'not', 'ap-', 'preciably', 'influence', 'its', 'activity', 'in', 'vitro”.', 'Actin-binding', 'proteins', 'such', 'as', 'profilin,', 'which', 'bind', 'to', 'Ptdins(4,5)P,', 'and', 'mask', 'it', 'from', 'the', 'action', 'of', 'non-phosphorylated', 'PLC-y,,', 'might', 'be', 'a.', 'key', 'intermediate', 'in', 'both', 'these', 'events.', 'It', 'also', 'seems', 'likely', 'that', 'other', 'actin-binding', 'proteins,', 'which', 'bind', 'PtdIns(4,5)P,,', 'such', 'as', 'gelsolin', 'and', 'pCap39,', 'might', 'similarly', 'transduce', 'sig-', 'nals', 'from', 'RTKs', 'to', 'the', 'actin', 'cytoskeleton.', 'An', 'alternative', 'mechanism', 'by', 'which', 'RTKs', 'may', 'influence', 'cytoskeletal', 'events', 'is', 'by', 'direct', 'action', 'of', 'the', 'receptors', 'on:', 'actin-binding', 'proteins', 'which', 'bear', 'Src', 'homology', '2', '(SH2).', 'domains.', 'For', 'example,', 'tensin,', 'an', 'actin-binding', 'protein', 'which', 'is', 'a', 'component', 'of', 'focal', 'contacts,,', 'bears', 'an', 'SH2', 'domain', 'that', 'binds', 'to', 'a', 'number', 'of', 'phosphotyrosine-containing', 'proteins™.', 'Tensin', 'may', 'therefore', 'bind', 'directly:', 'to', 'autophosphorylated', 'RTKs', 'and', 'mediate', 'their', 'direct', 'association', 'with', 'the', 'actin', 'cytoskeleton.', 'In', 'addition,', 'many', 'RTKs', 'activate', 'phospholipases', 'that', 'generate', 'diacylglycerol', 'and', 'InsP,;', 'these', 'two', 'sec-', 'TiBS', '17', '-', 'OCTOBER.', '1992', 'ond!', 'messengers,', 'in', 'turn,', 'activate', 'PKC', 'and', 'increase', 'intracellular', 'Ca?’.', 'As', 'dis-', 'cussed', 'above,', 'diacylglycerol:', 'can', 'nu-', 'cleate', 'actin', 'filaments’,', 'Ca?', 'can', 'regu-', 'late', 'a', 'large', 'number', 'of', 'actin-binding', "proteins',", 'and', 'PKC', 'can', 'alter', 'actin', 'struc-', 'ture', 'through', 'MARCKS?:', 'The', 'Ras-CAP', 'signal', 'transductton', 'pathway:', 'The', 'Ras', 'family', 'of', 'low', 'molecular', 'weight', 'guanine', 'nucleotide-binding', 'proteins', 'has', 'ai', 'role', 'in:', 'signal', 'transduction', 'pathways', 'that', 'regulate', 'growth', 'and', 'differentiation”!', '(also', 'see', 'other', 'reviews', 'ii', 'this', 'issue).', 'In', 'the', 'yeasti', 'Saccharomyces', 'cerevisiae,', 'Ras', 'activates.adenylate', 'cyclase', 'activity', 'in', 'a', 'complex', 'that', 'consists.', 'of', 'at', 'least', 'two', 'components:', 'the', 'adenylate', 'cyclase', 'cata-', 'lytic', 'subunit', 'and', 'an', 'adenylate', 'cyclase.', 'associated', 'protein', '(CAP).', 'CAP', 'is', 'a.', 'bifunctional', 'signal', 'transducer:', 'its', 'amino-', 'terminal', 'domain', 'is', 'necessary', 'and’', 'suf-', 'ficient.', 'for', 'cellular', 'responsiveness.', 'to:', 'activated', 'Ras,', 'and', 'its', 'carboxy-terminal', 'domain', 'has', 'a', 'role', 'in', 'a', 'number', 'of', 'cellu-', 'lar', 'functions', 'including', 'the', 'control', 'of', 'cell', 'morphology.', 'For', 'example,', 'yeast', 'cells', 'which', 'are', 'null', 'for', 'CAP', 'bud', 'randomly', 'and', 'are', 'defective', 'invactin', 'distribution”,', 'The', 'morphological:', 'and', 'nutritional', 'de-', 'fects', 'associated', 'with', 'the', 'loss', 'of', 'the', 'CAP', 'carboxy-terminal', 'domain', 'are', 'suppressed:', 'by', 'overexpression', 'of', 'the', 'profilin:', 'gene™.', 'In', 'addition,', 'cells', 'lacking', 'profilin.', 'are', 'abnormally', 'enlargediand', 'round,', 'do', 'not', 'contain', 'actin', 'cables', 'and', 'bud', 'randomly', 'from', 'the', 'cell', 'surface”,', 'a', 'phenotype', 'simi-', 'lar', 'to', 'that', 'of:cells', 'lacking', 'the', 'carboxy-', 'terminal', 'domain', 'of', 'CAP.', 'Mutational', 'analysis', 'and', 'studies', 'using', 'Acantha-', 'moeba', 'profilins', 'indicate', 'thatthe', 'ability', 'of', 'profilin', 'to', 'suppress', 'the', 'cap”', 'pheno-', 'type', 'is', 'dependent', 'on', 'the', 'ability', '‘to', 'bind:', 'Ptdins(4,5)P,', 'in', 'conjunction', 'with.', 'its', 'actin-binding', 'capacity.', 'Thus,', 'it', 'ap-', 'pears', 'that', 'CAP', 'and', 'profilin', 'are', 'compo-', 'nents', 'of', 'the', 'same', 'signaling:', 'pathway,', 'which', 'implies', 'that', 'CAP', 'may.', 'be-a', 'com-', 'ponent', 'of', 'a', 'polyphosphoinositide:', 'sig-', 'nal', 'transduction.', 'pathway.', 'Alterna-', 'tively,', 'CAP', 'itselfimay', 'bind', 'actin,', 'or', 'CAP', 'and', 'profilin', 'may', 'bind', 'to', 'each', 'other.', 'It', 'seems', 'likely', 'thati', 'CAP', 'itself', 'is', 'an.', 'actin-', 'binding', 'protein,', 'since', 'it', 'is', 'highly', 'hom:', 'ologous', 'to', 'ASP-56,.an', 'actin', 'monomer', 'sequestering', 'protein', 'from', 'porcine', 'platelets*.', 'However,', 'the', 'possibility', 'that', 'CAP', 'may', 'be', 'regulated', 'by', 'phospholipids', 'is', 'of', 'special!', 'interest', 'since', 'the', 'activity.', 'of', 'Ras', 'has', 'also', 'been', 'shown.to', 'be', 'regu-', 'lated', 'by', 'bioactive', 'phospholipids*.', 'Conclusions', 'and', 'perspectives', 'Advances', 'in', 'our', 'understanding', 'of', 'signal', 'transduction:', 'and', 'cytoskeletal', 'PZ8PTTEZO?']