Twin-arginine transport complex plays an essential role in <i>Caulobacter</i> cell shape and viability

Two main pathways are responsible for protein secretion across the cytoplasmic membrane in prokaryotes. While the general secretory (Sec) pathway transports proteins across the membrane in an unfolded state, the twin-arginine translocation (Tat) pathway exports proteins primarily in their folded conformation. Although the Tat system appears dispensable in multiple model bacteria, some species require it for viability, and the reason for the distinction is nebulous. Here we show that all three subunits of the Tat complex-TatA, TatB, and TatC-are essential in the alpha-proteobacterium <i>Caulobacter crescentus</i>. Additionally, depletion of the Tat complex results in abnormal cell morphology. We found that localization to the cell periphery, as well as midcell localization upon osmotic upshift, of the essential peptidoglycan transpeptidase PBP2 is dependent on the Tat apparatus. In contrast, subcellular localization of the actin homolog MreB and the penicillin-binding protein PBP1a is not perturbed upon depletion of the Tat complex. As PBP2 transpeptidase activity links glycan chains at sites of cell wall remodeling and is essential for cell elongation, localization results and leader sequence analysis together suggest that PBP2 translocation is a key responsibility of the Tat system in <i>Caulobacter</i> and possibly other alpha-proteobacteria.