Identification of a new isoform of the murine Sh2dla gene and its functional implications

Signaling lymphocytic activation molecule （SLAM）-associated protein （SAP） is a Src homology （SH） domain 2-containing in- tracellular adaptor protein that is predominantly expressed in the hematopoietic system by T lymphocytes and NK cells. SAP protein is encoded by the SH2DIA gene located on the X chromosome. Loss-of-function mutations in SAP cause the X-linked lymphoproliferative disease （XLP）, a severe immunodeficiency characterized by heightened susceptibility to Epstein-Barr vi- rus and impaired humoral immunity. Normal individuals express several functional and non-functional isoforms of SAP as a result of alternative splicing. In this study, we identify a cryptic exon in the murine Sh2dla gene. At the mRNA level, the new isoform of SAP （SAP-2） that includes this new exon is widely expressed in lymphoid tissues by C57BL/6 and 129 strains of inbred mice. SAP-2 accounts for approximately 1%-3% of total SAP transcripts, and it is dynamically regulated during lym- phocyte activation. At the protein level, the SAP-2 isoform is a 144 amino-acid protein. Compared to the dominant 126 ami- no-acid SAP-I isoform, the additional 18 amino acids are inserted into a structural region that is critical for phosphotyrosine binding. Our functional analysis in vitro indicates that SAP-2 is a non-functional isoform due to decreased protein stability. Thus, both human and mouse have multiple SAP splice isoforms that may or may not function. Modulation of relative propor- tions of these isoforms is potentially a mechanism whereby cells can regulate SAP-mediated biological activities.