A missense methionine mutation augments catalytic activity but reduces thermal stability in two protein tyrosine phosphatases.

Recent data sets that catalog the missense mutations in thousands of human genomes have revealed a vast and largely unexplored world of non-canonical protein sequences that are expressed in humans. The functional consequences of most human missense mutations, however, are unknown, and the accuracy with which their effects can be predicted by computational algorithms remains unclear. Among humans of European descent, the most common missense mutation in the catalytic domain of SH2-containing protein tyrosine phosphatase 1 (SHP-1) converts the enzyme's canonical valine 451 to methionine (V451M). The V451M mutation lies in a conserved motif adjacent to the protein tyrosine phosphatase (PTP) consensus sequence and is predicted to compromise catalytic function. In this study it is shown that, counter to prediction, V451M SHP-1 possesses increased catalytic activity as compared to the wild-type enzyme. Additionally, a PTP-wide search of missense-mutation data revealed a variant of one other PTP, Fas-associated PTP (FAP-1), that contains a methionine mutation at the position corresponding to 451 of SHP-1 (T2406M FAP-1). It is shown here that the T2406M mutation increases FAP-1's PTP activity, to a degree that is comparable to the activation deriving from the V451M mutation in SHP-1. Although the two non-canonical methionine residues confer increased activity at moderate temperatures, both V451M SHP-1 and T2406M FAP-1 are less thermally stable than their canonical counterparts, as demonstrated by the mutants' strongly reduced activities at high temperatures. These results highlight the challenges in predicting the functional consequences of missense mutations, which can differ under varying conditions, and suggest that, with regard to position 451/2406, canonical PTP domains have "chosen" stability over optimized activity during the course of evolution.