Protein secretion zones within the Gram-positive cell wall

Whereas the translocation of proteins across the cell membrane has been thoroughly investigated, it is still unclear how proteins cross the cell wall in Gram positive bacteria, which are widely used for industrial applications. We have studied secretion of -amylase AmyE within two different Bacillus strains. We show that a C-terminal fusion of AmyE with the fluorescent reporter mCherry is secreted via discrete patches with very low dynamics that are visible at many places within the lateral cell wall, for many minutes. Expression from a high copy number plasmid was required to be able to see these structures we term "secretion zones". Zones corresponded to visualized AmyE activity on the surface of cells, and frequently overlapped with SecA signals, but did not necessarily co-localize with the secretion ATPase. Single particle tracking showed much higher dynamics of SecA and of SecDF, involved in AmyE secretion, at the cell membrane than AmyE. These experiments suggest that SecA initially translocates AmyE molecules through the cell membrane, and then diffuses to a different translocon. Single molecule tracking of SecA show changes in distinct diffusion states of SecA, but suggest that AmyE overexpression does not overwhelm the system. Because secretion zones were only found during the transition to and within stationary phase, diffusion rather than passive transport based on cell wall growth from inside to outside, releases AmyE and thus probably secreted proteins in general. Our findings suggest active transport through the cell membrane and slow, passive transition through the cell wall in Gram positive bacteria.