Protein localization and dynamics within a bacterial organelle.

Protein localization mechanisms dictate the functional and structural specialization of cells. Of the four polar surface organelles featured by the dimorphic bacterium Caulobacter crescentus, the stalk, a cylindrical extension of all cell envelope layers, is the least well characterized at the molecular level. Here we apply a powerful experimental scheme that integrates genetics with high-throughput localization to discover StpX, an uncharacterized bitopic membrane protein that modulates stalk elongation and is sequestered to the stalk. In stalkless mutants StpX is dispersed. Two populations of StpX were discernible within the stalk with different mobilities: an immobile one near the stalk base and a mobile one near the stalk tip. Molecular anatomy provides evidence that (i) the StpX transmembrane domain enables access to the stalk organelle, (ii) the N-terminal periplasmic domain mediates retention in the stalk, and (iii) the C-terminal cytoplasmic domain enhances diffusion within the stalk. Moreover, the accumulation of StpX and an N-terminally truncated isoform is differentially coordinated with the cell cycle. Thus, at the submicron scale the localization and the mobility of a protein are precisely regulated in space and time and are important for the correct organization of a subcellular compartment or organelle such as the stalk.