Complex sentence: The p300-CBP coactivator family is composed of two closely related transcriptional co-activating proteins (or coactivators) p300 (also EP300 or E1A binding protein p300) and CBP (also CREB binding protein or CREB binding protein).
Split into simpler sentences: The p300-CBP coactivator family is composed of two closely related transcriptional co-activating proteins (or coactivators). The first member of this family is p300 (also EP300 or E1A binding protein p300) and the second is CBP (also CREB binding protein or CREB binding protein).