Characterization of the 55-Residue Protein Encoded by the 9S E1A mRNA of Species C Adenovirus

Abstract

Early region 1A (E1A) of human adenovirus (HAdV) has been the focus of over 30 years of investigation and is required for the oncogenic capacity of HAdV in rodents. Alternative splicing of the E1A transcript generates mRNAs encoding multiple E1A proteins. The 55-residue (55R) E1A protein, which is encoded by the 9S mRNA, is particularly interesting due to the unique properties it displays relative to all other E1A isoforms. 55R E1A does not contain any of the conserved regions (CRs) present in the other E1A isoforms. The C-terminal region of the 55R E1A protein contains a unique sequence compared to all other E1A isoforms, which results from a frameshift generated by alternative splicing. The 55R E1A protein is thought to be produced preferentially at the late stages of infection. Here we report the first study to directly investigate the function of the species C HAdV 55R E1A protein during infection. Polyclonal rabbit antibodies (Abs) have been generated that are capable of immunoprecipitating HAdV-2 55R E1A. These Abs can also detect HAdV-2 55R E1A by immunoblotting and indirect immunofluorescence assay. These studies indicate that 55R E1A is expressed late and is localized to the cytoplasm and to the nucleus. 55R E1A was able to activate the expression of viral genes during infection and could also promote productive replication of species C HAdV. 55R E1A was also found to interact with the S8 component of the proteasome, and knockdown of S8 was detrimental to viral replication dependent on 55R E1A.

Effects of Adenovirus Type 5 E1A Isoforms on Viral Replication in Arrested Human Cells

“…These proteins arise from differential splicing of the same transcript and differ only by the presence of an internal sequence of 46 amino acids in the larger protein. At a later point in infection, splicing produces three additional mRNA species, which encode for proteins of 217, 171 and 55 amino acids [ 2 – 4 ]. Sequence comparisons of the largest E1A proteins of several adenovirus serotypes identified four regions of sequence conservation, designated conserved regions (CR) 1, 2, 3 and 4 [ 5 – 9 ].…”

The adenovirus 55 residue E1A protein is a transcriptional activator and binds the unliganded thyroid hormone receptor

“…The exact role of the interaction of the E1A 55R proteins with the TR is not clear, although it is presumably related to transcriptional regulation occurring at later stages of infection, as this is when the E1A 55R protein is expressed (Miller et al, 2012). Given similarities between nuclear hormone receptors, it is also possible that E1A 55R could engage in functionally important interactions with corepressor domains of other members of this family of transcription factors.…”

“…However, splicing at late times during infection favours the production of the 9S E1A mRNA, which then predominates. Despite the fact that the 9S mRNA is the major E1A product at late times and was originally identified in 1978 (Berk & Sharp, 1978), little is known about the 55R E1A protein other than that it can promote viral gene expression and replication (Miller et al, 2012).…”

Inhibition of androgen receptor transactivation function by adenovirus type 12 E1A undermines prostate cancer cell survival

“…The early region 1A ( E1A ) of the adenovirus genome expresses several alternatively spliced mRNAs encoding various E1A isoforms . The two widely studied Ad5 E1A splice variants encode for the 289‐residue (herein referred to as E1A5) and the 243‐residue (e1a5) proteins.…”