Characterization of McuB, a periplasmic chaperone-like protein involved in the assembly of Myxococcus spore coat

The MXAN3885 to -3882 gene locus cluster (designated here mcuABCD) of Myxococcus xanthus encodes a member of the archaic chaperone-usher (CU) systems that functions in spore coat formation. We show here that McuD, a putative spore coat protein, affects cellular accumulation and cell surface localization of the spore coat protein McuA. We previously reported that genetic disruption of the putative usher McuC nearly eliminates surface display of McuA and show here that lack of the periplasmic chaperone-like protein McuB, which forms a complex with McuA, has a similar effect. Deletion mutation confirms that the G1 β strand of McuB is absolutely essential for the stability and secretion of McuA. Site-directed mutagenesis identified two additional alternating hydrophobic residues Ile113 and Val115, together with the highly conserved proline within the G1 strand, as critical residues for chaperone function. These findings suggest that the assembly proteins McuB and McuC mediate the transport of McuA onto the cell surface and that McuA may interact with another spore coat protein, McuD, for its secretion. Importantly, although our data argue that the M. xanthus CU system is likely to use the basic principle of donor strand complementation (DSC), as in the cases of classical CU pathways, to promote folding and stabilization of the structural subunit(s), the periplasmic chaperone McuB appears to exhibit structural variation in mediating chaperone-subunit interaction.