Heterogeneous N-acylation is a tissue- and species-specific posttranslational modification.

Heterogeneous N-terminal glycine acylation recently has been reported for two proteins involved in visual signal transduction. Similar N-acylations have typically involved only myristate; however, none of the previously examined proteins were isolated from retinas. To determine whether heterogeneous N-acylation is tissue-specific or protein sequence-specific, the N-terminal modifications of the catalytic subunit of cAMP-dependent protein kinase, partially purified from bovine retinas, heart, and brain tissues, were characterized. Using tandem mass spectrometry and liquid chromatography coupled directly to an electrospray mass spectrometer, we found only myristate at the N termini of catalytic subunits from brain and heart tissue, whereas the N termini of the retina-derived subunits were heterogeneously acylated in a manner similar to recoverin and transducin. Thus it appears that the nature of N-terminal glycine acylation is determined by the cell or tissue type in which it is located, and not by the sequence of the modified protein. We also examined the N-acylation of recoverin purified from human retinas, as well as transducin purified from frog retinas, to determine if heterogeneous acylation of retinal proteins is a uniquely bovine phenomenon. Interestingly, human recoverin was modified by the same family of fatty acids found on the bovine retinal proteins, while frog transducin was modified homogeneously not with myristate, but with a doubly unsaturated (C14:2) fatty acyl group.