cpTatB

cpTatB is a component of the chloroplast twin-arginine translocation (cpTat) pathway, involved in transporting folded proteins across the thylakoid membrane in chloroplasts. It is the chloroplast homolog of bacterial TatB and forms, together with cp TatC, a receptor complex that recognizes preproteins bearing twin-arginine signal peptides.

cpTatB is encoded in the nuclear genome and synthesized as a precursor containing a chloroplast transit peptide. The precursor is imported into the chloroplast stroma, where the transit peptide is removed and the mature cpTatB is embedded in the thylakoid membrane. The mature protein typically features a single transmembrane helix, with the remaining regions oriented toward the appropriate stromal or luminal sides to participate in receptor assembly. The exact topology is consistent with its role as a small, membrane-associated subunit of the cpTat translocase.

Functionally, cpTatB participates in recognizing and binding substrates that are targeted to the thylakoid lumen or membrane, in concert with cpTatC. The cpTat receptor complex mediates the transport of fully folded proteins, including some chloroplast enzymes and components of the photosynthetic apparatus, across the thylakoid membrane. In plants, cpTatB works together with cpTatC and, in some species, auxiliary factors such as Hcf106 to form a functional translocase. Mutations or alterations in cpTatB can reduce transport efficiency or alter substrate specificity, rather than completely abolishing cpTat pathway activity, indicating partial redundancy and compatibility with other translocon components.

Evolution and distribution of cpTatB are linked to the presence of chloroplasts in photosynthetic eukaryotes, with homologs identified across plants, algae, and related lineages. It shares evolutionary ties with bacterial TatB, reflecting a conserved origin of the Tat translocation system.