SAP is a transmembrane transport protein with six predicted transmembrane helices, with a hydrophilic domain between helices 3 and 4. This hyrodphobic region is highly variable among identified Gap-like (GPL, peptidoglycolipid, addressing protein) proteins and may be involved in substrate recognition. SAP also belongs to the LysE protein superfamily (Pfam:PF01810), whose members have been implicated in small molecule transport in bacteria. Other Gap proteins export metabolites across the cell membrane so it is possible that Sap specifically may be involved in transport of sulfolipid-1 across the membrane [1].

1: Elucidation and chemical modulation of sulfolipid-1 biosynthesis in Mycobacterium tuberculosis. Seeliger JC, Holsclaw CM, Schelle MW, Botyanszki Z, Gilmore SA, Tully SE, Niederweis M, Cravatt BF, Leary JA, Bertozzi CR; J Biol Chem. 2012;287:7990-8000. PMID:22194604