Sperm Lysozyme-Like Protein 1 (SLLP1), an intra-acrosomal oolemmal-binding sperm protein, reveals filamentous organization in protein crystal form

Abstract

Sperm Lysozyme-Like Protein 1 (SLLP1) is one of the lysozyme-like proteins predominantly expressed in mammalian testes that lacks bacteriolytic activity, localizes in the sperm acrosome, and exhibits high affinity for an oolemmal receptor, SAS1B. The crystal structure of mouse SLLP1 (mSLLP1) was determined at 2.15Å resolution. mSLLP1 monomer adopts a structural fold similar to that of chicken/mouse lysozymes retaining all four canonical disulfide bonds. mSLLP1 is distinct from c-lysozyme by substituting two essential catalytic residues (E35T/D52N), exhibiting different surface charge distribution, and by forming helical filaments approximately 75Å in diameter with a 25Å central pore comprised of six monomers per helix turn repeating every 33Å. Cross-species alignment of all reported SLLP1 sequences revealed a set of invariant surface regions comprising a characteristic fingerprint uniquely identifying SLLP1 from other c-lysozyme family members. The fingerprint surface regions reside around the lips of the putative glycan binding groove including three polar residues (Y33/E46/H113). A flexible salt bridge (E46-R61) was observed covering the glycan binding groove. The conservation of these regions may be linked to their involvement in oolemmal protein binding. Interaction between SLLP1 monomer and its oolemmal receptor SAS1B was modeled using protein-protein docking algorithms, utilizing the SLLP1 fingerprint regions along with the SAS1B conserved surface regions. This computational model revealed complementarity between the conserved SLLP1/SAS1B interacting surfaces supporting the experimentally-observed SLLP1/SAS1B interaction involved in fertilization.

Structural, Functional and Phylogenetic Analysis of Sperm Lysozyme-Like Proteins

“…Mouse SLLP1 (mSLLP1) crystal structure (2GOI) solved at 2.30 Å has also been reported to contain glycan binding groove similar to lysozyme [61]. Buffalo SLLP1 shared 73% sequence identity with mSLLP1.…”

“…Crystal structure analysis of mSLLP1 suggested the presence of glycan binding groove similar to that in lysozyme [61]; however, only partially conserved in the central cavity. The surface charges present in the groove also differed between the two structures.…”

“…In case of buffalo LYZL proteins, structural models showed binding with (NAG) 3 in two binding modes almost similar to those observed for non-testicular lysozymes. The mSLLP1 and human SLLP1 did not show significant binding with chitin or different glycans in a glycoarray [61]. The binding affinity of mSLLP1 and lysozyme also differed for glycan moieties.…”

A lysozyme with altered substrate specificity facilitates prey cell exit by the periplasmic predator Bdellovibrio bacteriovorus

“…6b). Interestingly, these two YW variants, α-lactalbumin 24 and mouse sperm lysozyme-like protein 25 , are both annotated as inactive enzymes. Hence, like DslA, these proteins may use the YW motif to block binding of acetylated GlcNAc peptidoglycan.…”

“…b Comparison of DslA to non-catalytic members of the lysozyme superfamily. The fold of DslA and residues E143, E154, and Y228-W229 are shown overlaid with equivalent residues from mouse α-lactalbumin (1NF5, gray)24 , mouse sperm lysozyme-like protein (4YF2, maroon)25 , and the active enzyme H. sapiens lysozyme (1LZS, yellow)18 . The YW motif on an active-site loop of DslA is shared with α-lactalbumin and sperm lysozyme-like protein and is equivalent to an AW motif in H. sapiens lysozyme.…”