Evolution of SH2 domains

SH2 domains are widespread in metazoans and choanoflagellates, along with their cognate tyrosine kinases, but are also found in several other lineages. The Spt6 gene is found in most eukaryotes, including plants and excavates and is known to bind pSer rather than pTyr. The structure confirms pSer specificity, and notes that it is intermediate between the two major classes (STAT and Src) of SH2 domain (2). Coupled with it's phylogeny, this suggests that it may be the primordial SH2 domain.

There is a restricted family of clear SH2 domains in plants (3), with two similar members in Arabidopsis (SHA and SHB), and one in most other vascular plants and moss. The large family of plant GRAS proteins has also been proposed to be STAT homologs and contain SH2 domains (4). However, there is no significant overall sequence similarity in our hands between GRAS and SH2, even using profile-profile comparisons, and secondary structure predictions (JPred) also suggest that the proposed domain in GRAS does not fold like an SH2.

References:

(1) Yoh, SM, Cho, H, Pickle, L, Evans, RM, Jones, KA (2007).The Spt6 SH2 domain binds Ser2-P RNAPII to direct Iws1-dependent mRNA splicing and export. Genes & Dev. 21: 160-74

(2) Dengl, S, Mayer, A, Sun, M, Cramer, P (2009) Structure and in Vivo Requirement of the Yeast Spt6 SH2 Domain. MB 289:211-25.

(3) Williams, JG, Zvelebil, M. (2004) SH2 domains in plants imply new signalling scenarios Trends in Plant Science 9: 161-163

(4) Richards, DE, Peng, J, Harberg, N (2000). Plant GRAS and metazoans STATs: one family? BioEssays 22:57-7.