Methanosarcinales S-layer Tile Protein

The presence of an S-layer in a member of the Methanosarcinales was determined in the 1980s by electron microscope (EM) studies examining the cell morphology of Methanosarcina mazei.

Under conditions of low osmolarity the S-layer is extensively decorated with a polysaccharide, termed methanochondroitin, and the cells tend to grow in multicellular aggregates.

The high degree of primary amino acid sequence identity between the N- and C-terminal DUF1608 domains (79% identical and 87% similar) allowed the homology modeling of the N-terminal DUF1608 amino acid sequence onto one of the DUF1608 domains in the crystallographic DUF1608 CTR dimer thus providing the first high-resolution model of an Archaeal S-layer protein.

A model for the quaternary structure of the M. acetivorans S-layer was proposed based on packing of the MA0829 CTR in a hexagonal lattice in one of the two obtained crystal forms (Protein Data Bank accession number 3U2G).

Lateral translation of the trimeric unit creates a flat 2-dimensional sheet that has features consistent with the molecular properties of hexagonal archaeal S-layers.

Whereas the protein constituents of lipid-based barriers, such as bacterial outer membranes, can be rapidly modified in response to physiological or environmental stimuli, the large pore sizes of the S-layer composed of MSTP protein subunits are presumably required to allow passage of molecules across a protective barrier whose molecular features are difficult to modify.

The structure of the C-terminal DUF1608 domain of the Methanosarcinales S-layer Tile Protein MA0829. A, the C-terminal DUF1608 domain of MA0829 with domain I colored red, domain II in blue, and the connector subdomain in yellow. B, the crystallographic dimer of the MA0829 CTR. C, a trimer of crystallographic CTR homodimers. Ammonium citrate ligands from the crystallization solution bound at intermolecular interfaces are shown in space filling representation. D, the hexagonal tiles formed from trimeric CTR homodimer assemblies. The primary, asymmetric, and trimer pores are indicated. E, a cutaway side view of the model of the MSTP S-layer (the representation in D is rotated 90° from the viewer). The position of the primary pore is indicated by arrowheads. Adapted from [ 10 ]