ARF proteins Monomeric GTPase in the Ras superfamily responsible for regulating both COPI coat assembly and clathrin coat assembly. (Table 15–5, p. 854)
ARP (actin-related protein) complex (Arp 2/3 complex) Complex of proteins that nucleates actin filament growth from the minus end.
arrestin   Member of a family of proteins that contributes to GPCR desensitization by preventing the activated receptor from interacting with G proteins and serving as an adaptor to couple the receptor to clathrin-dependent endocytosis. (Figure 15–42) astral microtubule In the mitotic spindle, any of the microtubules radiating from the aster which are not attached to a kinetochore of a chromosome.
asymmetric cell division Cell division in which some important molecule or molecules are distributed unequally between the two daughter cells, causing these cells to become different from each other.
ATM (ataxia telangiectasia mutated protein) Protein kinase activated by double-strand DNA breaks. If breaks are not repaired, ATM initiates a signal cascade that culminates in cell cycle arrest. Related to ATR.
ATP (adenosine 5′-triphosphate) Nucleoside triphosphate composed of adenine, ribose, and three phosphate groups. The principal carrier of chemical energy in cells. The terminal phosphate groups are highly reactive in the sense that their hydrolysis, or transfer to another molecule, takes place with the release of a large amount of free energy. (Figure 2–33)
ATP synthase (F1Fo ATPase) Transmembrane enzyme complex in the inner membrane of mitochondria and the thylakoid membrane of chloroplasts. Catalyzes the formation of ATP from ADP and inorganic phosphate during oxidative phosphorylation and photosynthesis, respectively. Also present in the plasma membrane of bacteria.
ATR (ataxia telangiectasia and Rad3 related protein) Protein kinase activated by DNA damage. If damage remains unrepaired, ATR helps initiate a signal cascade that culminates in cell cycle arrest. Related to ATM.
autoimmune disease Pathological state in which the body mounts a disabling adaptive immune response against one or more of its own molecules.
autophagosome   Organelle surrounded by a double membrane contains engulfed cytoplasmic cargo in the initial stages of autophagy.
autophagy   Digestion of cytoplasm and worn-out organelles by the cell’s own lysosomes.
auxin Plant hormone, commonly indole-3-acetic acid, with numerous roles in plant growth and development.
axon   Long nerve cell projection that can rapidly conduct nerve impulses over long distances so as to deliver signals to other cells.
axoneme   Bundle of microtubules and associated proteins that forms the core of a cilium or a flagellum in eukaryotic cells and is responsible for their movements.
bacterial artificial chromosome (BAC) Cloning vector that can accommodate large pieces of DNA, typically up to 1 million base pairs.
bacteriorhodopsin   Pigmented protein found in the plasma membrane of a salt-loving archaeon, Halobacterium salinarium (Halobacterium halobium). Pumps protons out of the cell in response to light.
bacterium (plural bacteria) (eubacterium) Member of the domain bacteria, one of the three main branches of the tree of life (archaea, bacteria, and eukaryotes). Bacteria and archaea both lack a distinct nuclear compartment, and together comprise the prokaryotes. (Figure 1–17)
Bak   A main effector Bcl2 family protein of the intrinsic pathway of apoptosis in mammalian cells that is bound to the mitochondrial outer membrane even in the absence of an apoptotic signal; activation is usually by activated pro-apoptotic BH3-only proteins.
basal Situated near the base. Opposite the apical surface.