Non-catalytic component of the 20S prmt5-containing methyltransferase complex, which modifies specific arginines to dimethylarginines in several spliceosomal Sm proteins and histones. Required for normal prmt5 methyltransferase activity. Heterotetramer; dimer of heterodimer with prmt5. Interacts with histone h2a and h4 and with nucleoplasmin. Enriched on chromatin. Detected in egg (at protein level).