Significance of hydrophobic and charged sequence similarities in sodium-bile acid cotransporter and vitamin D-binding protein macrophage activating factor
Sodium-bile acid cotransporter, also denominated sodium-taurocholate cotransporting polypeptide (NTCP) is an integral membrane protein with multiple hydrophobic transmembrane domains. The third extracellular domain of NTCP presents a stretch of nine aminoacids (KGIVISLVL) that is characterized by pronounced hydrophobicity and serves as receptor for a protein, preS1, showing the hydrophobic epta-peptide sequence NPLGFFP. Vitamin D-binding protein macrophage activating factor (DBP-MAF) is a multifunctional protein that is characterized by two hydrophobic regions able to bind fatty acids and vitamin D, respectively. Here we demonstrate that NTCP and DBP-MAF show significant sequence similarities as far as hydrophobic stretches of aminoacids are concerned. Alignment of the sequence of seven aminoacids preceding the 157-KGIVISLVL-165 stretch of NTCP shows four aminoacids that are identical to those of the corresponding sequence of DBP-MAF, and two that are conserved substitutions. In addition, in the sequence of DBP-MAF that is aligned with the sequence YKGIVISLVL of NTCP, there are two contiguous negatively charged aminoacids (ED) and, in the preceding epta-peptide sequence, there are three negatively charged aminoacids (D-ED), whereas in the corresponding sequence of NTCP there are only two (D--D) that are not contiguous. This concentration of negatively charged aminoacids may be involved in binding of protein inserts characterized by high density of positively charges residues. The alternating hydrophobic and electrostatic interactions described in this paper may help elucidating the biological roles of these proteins as far as protein-protein interactions are concerned.