Complex sentence: There are several "families" of E3 ubiquitin ligases; these families are comprised by members that have particular protein domains that are capable of binding the E2 conjugase, and thus forming a ubiquitination complex.
Split into simpler sentences: There are several "families" of E3 ubiquitin ligases. These families are comprised by members that have particular protein domains that are capable of binding the E2 conjugase, and thus forming a ubiquitination complex.