Based on the study in mouse (Uechi et al. 2014), the spermatoproteasome-specific alpha (outer) ring subunit alpha-4s (PSMA8) co-immunoprecipitates components of the 19S regulatory particle (19S RP). Based on the study using bovine proteins, 19S RP is the predominant regulatory particle that is present in spermatoproteasomes (Zivkovic et al. 2022). In both mouse (Uechi et al. 2014) and bovine (Ustrell et al. 2002; Zivkovic et al. 2022) testis, a component of the spermatoproteasome is a spermatoproteasome-specific proteasome activator PSME4 (PA200).

The biological role of spermatoproteasomes has not been fully elucidated, but the spermatoproteasome-specific alpha-4s (PSMA8) subunit of the 20S outer ring is needed for the proper meiotic exit and fertility in male mice (Gomez-H et al. 2019). Based on the analogy with the formation of the 26S proteasome (reviewed in Tomko and Hochstrasser 2013), it is assumed that 19S RP chaperones PAAF1 (Rpn14), PSMD10 (Nas6), and PSMD5 (Hsm3) are released from the 19S RP precursor upon binding to the 20S spermatoproteasome core particle (20S sCP).