Structure of the lysosomal sphingolipid activator protein 1 by homology with influenza virus neuraminidase

The sphingolipid activator protein 1 (SAP-1) increases the rate of hydrolysis of sphingolipids in the lysosome by apparently bringing together the substrate and the corresponding hydrolytic enzyme. This implies specific recognition of both the substrate and enzyme by SAP-1. However, binding domains in SAP-1 and recognition mechanisms involved are unknown. Amino acid sequence comparison of SAP-1 with influenza virus neuraminidase (EC 3.2.1.18, FLU NA) indicates that functional amino acid residues in or near the sialic acid binding site of FLU NA are also found at equivalent positions in the first 48 N-terminal amino acids of SAP-1. This region of homology allows to propose folding of the SAP-1 polypeptide chain by comparison with known crystallographic structure of FLU NA and identify a potential domain for lysosomal enzyme recognition through sialic acid binding. There is also a region of 10 amino acid residues near the C-terminal end of SAP-1 which has a strong propensity to form an alpha-helix with amphiphilic properties of lipid-binding helices. This domain in SAP-1 is probably responsible for the lipid(substrate)-binding function of SAP-1.