This is the Acidic Tract of the name FFAT, and it is mainly found amino-terminal to the core motif, but also extends to the carboxy-terminal side to some extent.
[3] This led to a simple hypothesis that VAP directly binds FFAT motifs, which was tested by biochemical interaction between purified components,[1] and was later confirmed by structural analysis of VAP-FFAT complexes, both by X-ray crystallography[4] and by NMR.
[7] Many of the proteins with FFAT motifs were previously not known to be targeted to the endoplasmic reticulum, with the exception of OSBP,[3] and PITPNM1 (the fly homologue of which is called RdgB).
[4] Subsequently, other proteins have been found in variants of FFAT motifs with quite divergent residues, including K (lysine) at position 3 in protrudin.
[9] An attempt was made to rank FFAT-like sequences by scoring substitutions at all 6 positions of the core motif and the number of nearby acidic residues (DEST).
[11] This indicates that cAMP signalling is yet another cellular activity involving small molecules that is regulated at membrane contact sites, along with lipid and calcium ion traffic.