A lack of peptide binding and decreased thermostability suggests that the CASKIN2 scaffolding protein SH3 domain may be vestigial.
BACKGROUND
CASKIN2 is a neuronal signaling scaffolding protein comprised of multiple ankyrin repeats, two SAM domains, and one SH3 domain. The CASKIN2 SH3 domain for an NMR structural determination because its peptide-binding cleft appeared to deviate from the repertoire of aromatic enriched amino acids that typically bind polyproline-rich sequences.


RESULTS
The structure demonstrated that two non-canonical basic amino acids (K290/R319) in the binding cleft were accommodated well in the SH3 fold. An K290Y/R319W double mutant restoring the typical aromatic amino acids found in the binding cleft resulted in a 20 °C relative increase in the thermal stability. Considering the reduced stability, we speculated that the CASKIN2 SH3 could be a nonfunctional remnant in this scaffolding protein.


CONCLUSIONS
While the NMR structure demonstrates that the CASKIN2 SH3 domain is folded, its cleft has suffered two substitutions that prevent it from binding typical polyproline ligands. This observation led us to additionally survey and describe other SH3 domains in the Protein Data Bank that may have similarly lost their ability to promote protein-protein interactions.
['Kwan|Jamie J|JJ|', 'Donaldson|Logan W|LW|']
D048868:Adaptor Proteins, Signal Transducing D000595:Amino Acid Sequence D006801:Humans D008958:Models, Molecular D009419:Nerve Tissue Proteins D019906:Nuclear Magnetic Resonance, Biomolecular D010455:Peptides D011485:Protein Binding D017510:Protein Folding D055550:Protein Stability D013696:Temperature D018909:src Homology Domains
SH3 domain Molecular modeling NMR spectroscopy