SHP-1 variant proteins are absent in motheaten mice despite presence of splice variant transcripts with open reading frames

Motheaten mice have a mutation that causes abnormal splicing of the SHP-1 gene producing transcripts that are out of frame. Thus, motheaten mice cannot produce normal SHP-1 protein. However, we have found that the SHP-1 locus in normal mice is expressed as multiple in-frame splice variant transcripts. We report here that the motheaten SHP-1 gene is likewise expressed in multiple spliced forms, two of which are in frame. One of these two variants, which is also present in normal mice, lacks the exon containing the motheaten mutation and is therefore expected to encode an active phosphatase with only one of the two SH2 domains of SHP-1. We showed that both of these variants produce phosphatases with a higher specific activity than SHP-1, suggesting that motheaten mice are not SHP-1 null. The possibility that motheaten mice produce disregulated phosphatases offered a simple explanation for the puzzling observation that substrates of SHP-1 are hypo-phosphorylated in motheaten mice. We tested this by measuring for SHP-1 protein and activity in motheaten macrophages. However, we did not detect specific activity, and found that one of these variant proteins was unstable. These findings likewise suggest that little or no SHP-1 variant proteins exist in normal cells. (C) 2000 Elsevier Science Ltd. All rights reserved.