The structure model of this domain resembles the C-terminal domains of the Streptococcus intermedius antigen I/II (AgI/II). The N-terminal domain of AgI/II (also called adhesin P1, antigen B or PAc) folds towards the stalk, where it gets locked in place [1]. Given that this domain is found N-terminal to repeating stalk domains in bacterial surface proteins, the domain is suggested to have a potential protein binding function.

1: An intramolecular lock facilitates folding and stabilizes the tertiary structure of Streptococcus mutans adhesin P1. Heim KP, Crowley PJ, Long JR, Kailasan S, McKenna R, Brady LJ; Proc Natl Acad Sci U S A. 2014;111:15746-15751. PMID:25331888