Complex sentence: The first subunit, which was identified by yeast genetics is Sec11, a 17 kDa membrane protein that is associated with three subunits termed Spc3p (21 kDa), Spc2p (18 kDa) and Spc1p (11 kDa).
Split into simpler sentences: The first subunit, which was identified by yeast genetics, Sec11 that is a 17 kDa membrane protein. Sec11 is associated with three subunits termed Spc3p (21 kDa), Spc2p (18 kDa) and Spc1p (11 kDa).