Fig. 4: Hypothetical models for thread assembly.

Saci_0406 precursor proteins are initially secreted through the SEC translocon (1). Signal peptidase 1 (SP1) cleaves the N-terminal signal peptide (2). The protein is glycosylated by AglB (3; glycans shown as black sticks). Soluble, mature Saci_0406 proteins assemble into the thread via donor strand complementation (4, 5). This process may be spontaneous or catalysed by an assembly machinery. The putative cap protein Saci_0405 is not predicted to be secreted through SEC. Its location in the thread depends on the polarity of the filament, which is currently unknown. If the Saci_0406 tails point away from the cell, Saci_0405 forms a cell proximal cap that terminates thread assembly (hypothesis 1). If the Saci_0406 tails point toward the cell, Saci_0405 forms a cell distal cap, which initiates thread assembly (hypothesis 2).