Protein Export by the Mycobacterial SecA2 System Is Determined by the Preprotein Mature Domain

Abstract

At the core of the bacterial general secretion (Sec) pathway is the SecA ATPase, which powers translocation of unfolded preproteins containing Sec signal sequences through the SecYEG membrane channel. Mycobacteria have two nonredundant SecA homologs: SecA1 and SecA2. While the essential SecA1 handles "housekeeping" export, the nonessential SecA2 exports a subset of proteins and is required for Mycobacterium tuberculosis virulence. Currently, it is not understood how SecA2 contributes to Sec export in mycobacteria. In this study, we focused on identifying the features of two SecA2 substrates that target them to SecA2 for export, the Ms1704 and Ms1712 lipoproteins of the model organism Mycobacterium smegmatis. We found that the mature domains of Ms1704 and Ms1712, not the N-terminal signal sequences, confer SecA2-dependent export. We also demonstrated that the lipid modification and the extreme N terminus of the mature protein do not impart the requirement for SecA2 in export. We further showed that the Ms1704 mature domain can be efficiently exported by the twin-arginine translocation (Tat) pathway. Because the Tat system exports only folded proteins, this result implies that SecA2 substrates can fold in the cytoplasm and suggests a putative role of SecA2 in enabling export of such proteins. Thus, the mycobacterial SecA2 system may represent another way that bacteria solve the problem of exporting proteins that can fold in the cytoplasm.

The Non-catalytic “Cap Domain” of a Mycobacterial Metallophosphoesterase Regulates Its Expression and Localization in the Cell

“…We assessed the localization of Rv0805 in a strain harboring a deletion in the secA2 gene, coding for a chaperone shown to assist in the secretion of client proteins that do not contain a canonical signal peptide (27,28). However, localization of Rv0805 to the cell membrane was unaffected in this strain (Fig.…”

Structural Similarities and Differences between Two Functionally Distinct SecA Proteins, Mycobacterium tuberculosis SecA1 and SecA2

“…The absence of the HWD in SecA2 makes the cleft significantly more open and solvent exposed (illustrated in Fig. S3 in the supplemental material), which could help SecA2 recognize its unique substrates that are distinguished by features of their mature domains-possibly a tendency to fold prior to export (23).…”

“…Interestingly, when fused to a signal peptide for the twin-arginine translocation (Tat) pathway, the mature domain of Ms1704 is exported by the Tat pathway. This result suggests that the defining feature of SecA2 substrates may be a tendency to fold prior to export (23). This is because proteins that get translocated across the membrane by the Tat pathway must be folded in the cytoplasm prior to export (24).…”

“…Of the signal peptide-containing proteins exported by the SecA2 systems of Mycobacterium smegmatis (22), M. marinum (10), and M. tuberculosis (21), the most thoroughly studied proteins are the M. smegmatis 1704 (Ms1704) and Ms1712 proteins (22). Studies of Ms1704 and Ms1712 demonstrate that they require their signal peptide for export, but it is a feature of the mature portions of these proteins that necessitates export via the SecA2-dependent pathway (23). Interestingly, when fused to a signal peptide for the twin-arginine translocation (Tat) pathway, the mature domain of Ms1704 is exported by the Tat pathway.…”

Bacillus anthracis SlaQ Promotes S-Layer Protein Assembly

“…SecA2 was first identified in Mycobacterium tuberculosis, where it promotes the transport of proteins across the mycobacterial envelope that aid in the pathogenesis of tuberculosis (44,45). Mycobacterial SecA2 recognizes the mature domains of a few secretory precursors to assist in their secretion, suggesting its primary function may be maintenance of export competence for proteins with the propensity for rapid folding (46). Listeria monocytogenes secA2 was identified because mutations in the structural gene affect colony shape and cellular morphology, a phenotype that is attributed to the diminished secretion of cell wall hydrolases (i.e., p60 autolysin, NamA hydrolase, and two dozen other substrates) (47)(48)(49).…”