Isoform 2 inhibits the ubiquitin ligase activity of SIAH1 and inhibits proteasomal degradation of target proteins. Isoform 2 inhibits autoubiquitination and proteasomal degradation of SIAH1, and thereby increases cellular levels of SIAH. Isoform 2 modulates SNCA monoubiquitination by SIAH1. {ECO:0000269|PubMed:16595633, ECO:0000269|PubMed:19224863}. This is the function of Ensembl gene identifier ENSG00000064692 (SNCAIP, synuclein alpha interacting protein).