Surface-layer proteins (SLPs) play a crucial role in the self-assembly of bacterial surface layers, yet the structural details of their assembly domains remain largely unexplored. Here, we report the crystal structure of SlpM_C1, a structural module within the self-assembly domain of SlpM from Lactobacillus brevis. SlpM_C1 adopts a β-grasp fold, a conserved structural motif found in diverse protein families. Structural comparisons with ubiquitin and the SlpA_II domain from L. acidophilus reveal both shared and distinct features, highlighting elements of structural convergence despite sequence divergence. Furthermore, the dimerization patterns of SlpM_C1 and SlpA_II are compared and discussed. These findings provide new insights into the architecture and evolutionary adaptability of SLPs in Lactobacillus species.