The function of P‐selectin glycoprotein ligand‐1 is conserved from ancestral fishes to mammals

PSGL‐1 is a mucin‐like glycoprotein that supports, in mammals, leukocyte rolling on selectins. However, we have limited knowledge whether its function is conserved in non‐mammals and how its structure adapted during evolution. To identify conserved amino acid sequences required for selectin binding, we performed multiple alignments of PSGL‐1 sequences from 18 mammals, 4 birds, 3 reptiles, 1 amphibian, and 15 fishes. The amino‐terminal T[D/E]PP[D/E] motif, which identifies in mammals a core‐2 O‐glycosylated threonine required for selectin‐binding, is partially conserved in some fishes (e.g., T. rubripes) and birds (e.g., G. gallus), however, most non‐mammals do not display it. The sulfated tyrosine residues of human PSGL‐1, which bind L‐ and P‐selectin, are not observed in non‐mammals, suggesting that they are dispensable for selectin‐binding or that other amino acids play their role. A mucin‐like domain is present in all species. Interestingly, the alignment of cytoplasmic sequences of non‐mammals reveals the conservation of ezrin/radixin/moesin binding site and two new motifs (M1 and M2). To examine the conservation of PSGL‐1 function, we cloned PSGL‐1 cDNA sequences of zebrafish and fugu, and established their cross‐reactivity with human selectins under flow conditions. Importantly, deleting the well‐conserved M1 motif strongly decreased PSGL‐1 expression at leukocyte surface and induced retention of the precursor molecule in the endoplasmic reticulum, indicating that M1 motif provides a signal required to export PSGL‐1 precursors to the Golgi complex. These data show for the first time the conservation of PSGL‐1 function from fishes to mammals and reveal the function of a new motif. PSGL‐1 function is evolutionarily conserved from ancestral fishes to mammals: This study identifies new highly conserved sequence motifs.