Hyperthermostable surface layer protein tetrabrachion from the archaebacterium Staphylothermus marinus: evidence for the presence of a right-handed coiled coil derived from the primary structure.

The scaffold of the surface layer covering the hyperthermophilic archaebacterium Staphylothermus marinus is formed by an extended filiform glycoprotein complex, tetrabrachion, which is anchored in the cell membrane at one end of a 70 nm stalk and branches at the other end into four arms of 24 nm length. The arms from a canopy-like meshwork by end-to-end contacts, enclosing a "quasi-periplasmic space". The primary structure of the complex, obtained by an approach based entirely on the polymerase chain reaction, shows that the light and the heavy chains are encoded in this order in a single gene and are generated by internal proteolytic cleavage. One light chain associates with the N-terminal part of a heavy chain to form one of the four arms of the complex, comprising about 1000 residues. Following a glycine-rich linker of about ten residues, the C-terminal 500 residues of the four heavy chains converge to form a four-stranded parallel coiled coil, which ends in a transmembrane segment. The sequence of the coiled coil is exceptional in that the heptad repeat of hydrophobic residues typical for left-handed coiled coils shifts to an undecad repeat after an internal proline residue, indicating that the C-terminal part of the sequence forms a right-handed coiled coil. Such a periodicity has not been detected in coiled coils to date. The almost flawless pattern of aliphatic residues, mainly leucine and isoleucine, throughout the hydrophobic core of the stalk provide one explanation for its exceptional stability.