Involved in splicing regulation. Facilitates post-transcriptional gene silencing (PTGS) by limiting the degradation of transgene aberrant RNAs by the RNA quality control (RQC) machinery, thus favoring their entry into cytoplasmic siRNA bodies where they can trigger PTGS. Does not participate in the production of small RNAs. No visible phenotype (PubMed:26842463). Smd1a and smd2b double mutants are embryo lethal (PubMed:26842463). SMD1A and SMD1B have redundant activity, but consistent with their expression level, SMD1B is more important than SMD1A and either one copy of SMD1B or two copies of SMD1A is necessary for the plant to survive. Belongs to the snRNP core protein family.