Abstract
The L6 region of bovine adenovirus (BAdV)-3 encodes a spliced protein designated 33K. The 33K specific sera detected five major proteins and three minor proteins in transfected or virus infected cells, which could arise by internal initiation of translation and alternative splicing. The 33K protein is predominantly localized to the nucleus of BAdV-3 infected cells. The 33K nuclear transport utilizes both classical importin-α/-β and importin-β dependent nuclear import pathways and preferentially binds to importin-α5 and transportin-3 receptors, respectively. Analysis of mutant 33K proteins demonstrated that amino acids 201–240 of the conserved C-terminus of 33K containing RS repeat are required for nuclear localization and, binding to both importin-α5 and transportin-3 receptors. Interestingly, the arginine residues of conserved RS repeat are required for binding to transportin-3 receptor but not to importin-α5 receptor. Moreover, mutation of arginines residues of RS repeat proved lethal for production of progeny virus. Our results suggest that arginines of RS repeat are required for efficient nuclear transport of 33K mediated by transportin-3, which appears to be essential for replication and production of infectious virion.
Highlights
Nuclear transport of proteins usually takes place by binding of protein nuclear localization signal (NLS) to carrier proteins located in the cytoplasm [1]
The classical nuclear import is mediated by importin-a/-b pathway where protein NLS is recognized by an adapter protein importin-a, which interacts with actual transport receptor importin-b forming a heterodimer complex which is translocated through nuclear pore complex
We demonstrated that bovine adenovirus (BAdV)-3 33K protein a) is a product of spliced transcript, which shares N-terminus 138 amino acids with 22K, b) interact with BAdV-3 pV and 100K protein, c) may be involved in virus assembly, and, d) N-terminus 97 amino acids shared by 33K and 22K may be involved in DNA encapsidation [8,15,16]
Summary
Nuclear transport of proteins usually takes place by binding of protein nuclear localization signal (NLS) to carrier proteins located in the cytoplasm [1]. Most transport is mediated by karyopherins belonging to importin-b family including transportins (1,2, SR1, SR2), importins (4,5,7) and importin-b [2]. The classical nuclear import is mediated by importin-a/-b pathway where protein NLS is recognized by an adapter protein importin-a, which interacts with actual transport receptor importin-b forming a heterodimer complex which is translocated through nuclear pore complex. Some of the proteins do not require adapter proteins (importin-a) and directly bind the import receptor (importin-b, importin-7, transportin) for translocating through nuclear pore complex for transport to nucleus [1,2]. Some viral proteins use both classical importina/-b mediated and importin-b family members mediated nuclear import pathways [3,4,5,6]
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