Iterative structural homology search identifies new substrates of the protein O -fucosyltransferases POFUT3 and POFUT4

O -fucosylation plays an essential role in controlling protein folding, secretion and protein-protein interactions within the extracellular space. Recently, we identified a new form of protein O -fucosylation occurring on the N-terminal Elastin Microfibril Interaction (EMI) domain of several secreted proteins, mediated by two previously uncharacterized protein O -fucosyltransferases, POFUT3 (FUT10) and POFUT4 (FUT11). As all POFUT enzymes (POFUT1-4) are highly specific for the three-dimensional (3D) structure of their substrate protein domains, we postulated that structural homologues of these domains in other proteins may also be O -fucosylated. Here, we employed iterative protein structural homology searches as a novel strategy for identifying EMI-like domains that may serve as potential substrates for POFUT3/4. We discovered that microfibrillar-associated protein 2 and 5 (MFAP2/MFAP5) contain EMI-like domains and are O -fucosylated at high stoichiometry in human tissues. Unexpectedly, we showed that only POFUT3 is both necessary and sufficient for MFAP2/MFAP5 O -fucosylation, despite POFUT4 also having strong protein-protein interactions with MFAP2/MFAP5. Finally, we determined that O -fucosylation of MFAP2/MFAP5 is required for their efficient secretion, similar to other EMI domain-containing proteins. Together, these data demonstrate the power of sensitive structural homology analysis in identifying new enzyme-substrate relationships and protein-protein interactions.