A novel coiled-coil repeat variant in a class of bacterial cytoskeletal proteins.

In recent years, a number of bacterial coiled-coil proteins have been characterised which have roles in cell growth and morphology. Several have been shown to have a cytoskeletal function and some have been proposed to have an IF-like character in particular. We recently demonstrated in Streptomyces coelicolor a cytoskeletal role of Scy, a large protein implicated in filamentous growth, whose sequence is dominated by an unusual coiled-coil repeat. We present a detailed analysis of this 51-residue repeat and conclude that it is likely to form a parallel dimeric non-canonical coiled coil based on hendecads but with regions of local underwinding reflecting highly periodic modifications in the sequence. We also demonstrate that traditional sequence similarity searching is insufficient to identify all but the close orthologues of such repeat-dominated proteins, but that by an analysis of repeat periodicity and composition, remote homologues can be found. One clear candidate, despite a great size discrepancy and unremarkable sequence identity, is the known filament-former FilP in the same species. Both proteins appear distinct from the archetypal bacterial IF-like protein; they therefore may constitute a new class of bacterial filamentous protein. The similar sequence characteristics of both suggest their likely oligomer state and a possible mechanism for higher-order assembly into filaments. Another remote homologue in Actinomyces was highlighted by this method. Further, a known coiled-coil protein, DivIVA, appears to share some of these sequence characteristics.