Physicochemical characteristics of the mineral-forming S-layer from the cyanobacterium Synechococcus strain GL24

The hexagonal S-layer from the unicellular cyanobacterium Synechococcus strain GL24 can act as a template for sulfate and carbonate mineral formation by providing regularly spaced, chemically identical nucleation sites. This novel ability is a reflection of the S-layer's chemistry in addition to its structure. Few, if any, other cyanobacterial S-layers have been subjected to detailed chemical analysis and no other reports of the involvement of an S-layer in mineral formation can be found. We have investigated the surface charge and chemical characteristics of the Synechococcus strain GL24 S-layer in an attempt to unravel the mechanism(s) of mineral formation on this proteinaceous surface layer. The constituent proteins of the S-layer show up as two bands (Mr = 109 000 and 104 000) on SDS-PAGE gels. When assembled, the S-layer confers a net neutral charge on the cell surface with negatively charged sites (which can bind cations) exposed in the large holes of the array. Treatment of whole cells with a wide range of chemical agents failed to disrupt intralayer bonding and binding of the S-layer to the underlying outer membrane, indicating that this is a remarkably resilient S-layer in comparison to those found on other bacteria.