This is the C-terminal SH3-like domain which can be found in the exoribonuclease Xrn1. Xrn1 is a 175 kDa processive exoribonuclease that is conserved from yeast to mammals which targets cytoplasmic RNA substrates marked by a 5' monophosphate for processive 5'-to-3' degradation. The Sh3-like domain in Xrn1 lacks the canonical SH3 residues normally involved in binding proline-rich peptide motifs and instead engages in non-canonical interactions with the catalytic domain. Additionally it is essential in maintaining the structural integrity of Xrn1, since partial truncation of this domain in yeast Xrn1 yields an inactive protein. There is a long loop projecting from the SH3-like domain that contacts the PAZ/Tudor domain, occluding the functional surface that binds RNA or peptide motifs containing methylated arginines, respectively, in canonical PAZ and Tudor domain [1].

1: Coupled 5' nucleotide recognition and processivity in Xrn1-mediated mRNA decay. Jinek M, Coyle SM, Doudna JA; Mol Cell. 2011;41:600-608. PMID:21362555

2: Structural and biochemical studies of the 5'-->3' exoribonuclease Xrn1. Chang JH, Xiang S, Xiang K, Manley JL, Tong L; Nat Struct Mol Biol. 2011;18:270-276. PMID:21297639