Although the signal sequences for import into chloroplasts superficially resemble those for import into mitochondria, the same plant cells have both mitochondria and chloroplasts, so proteins must partition appropriately between the two organelles. In plants, for example, a bacterial enzyme can be directed specifically to mitochondria if it is experimentally joined to an N-terminal signal sequence of a mitochondrial protein; the same enzyme joined to an N-terminal signal sequence of a chloroplast protein ends up in chloroplasts. Thus, the import receptors on each organelle distinguish between the different signal sequences.
Chloroplasts have an extra membrane-enclosed compartment, the thylakoid. Many chloroplast proteins, including the protein subunits of the photosynthetic system and of the ATP synthase (discussed in Chapter 14), are located in the thylakoid membrane. Like the precursors of some mitochondrial proteins, the precursors of these proteins are translocated from the cytosol to their final destination in two steps. First, they pass across the double membrane into the matrix space (called the stroma in chloroplasts), and then they either integrate into the thylakoid membrane or translocate into the thylakoid space (Figure 12–26A). The precursors of these proteins have a hydrophobic thylakoid signal sequence following the N-terminal chloroplast signal sequence. After the N-terminal signal sequence has been used to import the protein into the stroma, a stromal signal peptidase removes it, unmasking the thylakoid signal sequence that initiates transport
Figure 12–26 Translocation of chloroplast precursor proteins into the thylakoid space. (a) The precursor protein contains an n-terminal chloroplast signal sequence (red), followed immediately by a thylakoid signal sequence (brown). The chloroplast signal sequence initiates translocation into the stroma by a mechanism similar to that used for the translocation of mitochondrial precursor proteins into the matrix space, although the translocator complexes, ToC and TiC, are different. The signal sequence is then cleaved off, unmasking the thylakoid signal sequence, which initiates translocation across the thylakoid membrane.
(B) Translocation into the thylakoid space or thylakoid membrane can occur by any one of at least four routes: (1) a Sec pathway, so called because it uses components that are homologs of sec proteins, which mediate protein translocation across the bacterial plasma membrane (discussed later); (2) an SRP-like pathway, so called because it uses a chloroplast homolog of the signal-recognition particle, or srp (discussed later); (3) a TAT (twin arginine translocation) pathway, so called because two arginines are critical in the signal sequences that direct proteins into this pathway, which depends on the h+ gradient across the thylakoid membrane; and (4) a spontaneous insertion pathway that seems not to require any protein translocator.
across the thylakoid membrane. There are at least four routes by which proteins cross or become integrated into the thylakoid membrane, distinguished by their need for different stromal chaperones and energy sources (Figure 12–26B).