The retrovirus MA and PreTM proteins follow immature MLV cores

•Analysis of detergent-resistant Murine Leukemia Virus core.•The matrix protein MA binds to immature MLV cores.•No p12 binds to immature MLV cores.•Nucleocapsid protein is released from the immature MLV core by RNase A.•High level of PreTM but low level of TM follows the immature MLV core.

We have used mild detergent to analyze the core of Moloney Murine Leukemia Virus (MoMLV) and core-like complexes in infected cells. The immature core consists of the Gag polyprotein (PrGag) and viral RNA (vRNA). It is known to be detergent-resistant, in contrast to the mature Gag core. The core matures by cleavage of PrGag into MA (matrix), p12, CA (capsid) and NC (nucleocapsid) protein. We found that mature Gag proteins were bound to the PrGag cores. The degree of binding differed widely. No (<0.1%) p12 bound, low amount of CA (3-5%), and higher amount of MA (13-20%) bound. Varying NC was bound (5-15%). NC could be released by RNase A in agreement with its binding to viral RNA. The TM (transmembrane) protein was also examined. A low amount of TM was bound to the PrGag core (approximately 5%), whereas a very high amount (65%) of the PreTM (TM with the cytoplasmic R peptide tail) bound. The binding in the PrGag core appears to occur by direct protein–protein interactions as only minute amounts of lipids including raft lipids were observed after detergent treatment.