Published online 15 September 2005
Article |
Viral mutations enhance the Max binding properties of the vMyc b-HLH-LZ domain
Biomedical Research Centre, University of Dundee, Ninewells Hospital and Medical School Dundee, DD1 9SY, UK 1Marie Curie Research Institute The Chart, Oxted, Surrey, RH8 OHL, UK 2VLA Laboratories New Haw, Addlestone, Surrey KT15 3NB, UK 3Beatson Institute for Cancer Research Garscube Estate, Switchback Road, Bearsden, Glasgow G61 1BD, UK
*To whom correspondence should be addressed. Tel: +44 1382 660111 ext. 33740; Fax: +44 1382 669993; Email: d.h.crouch{at}dundee.ac.uk
Received June 9, 2005. Revised August 16, 2005. Accepted August 27, 2005.
Interaction with Max via the helix–loop–helix/leucine zipper (HLH-LZ) domain is essential for Myc to function as a transcription factor. Myc is commonly upregulated in tumours, however, its activity can also be potentiated by virally derived mutations. vMyc, derived from the virus, MC29 gag-Myc, differs from its cellular counterpart by five amino acids. The N-terminal mutation stabilizes the protein, however, the significance of the other mutations is not known. We now show that vMyc can sustain longer deletions in the LZ domain than cMyc before complete loss in transforming activity, implicating the viral mutations in contributing to Myc:Max complex formation. We confirmed this both in vitro and in vivo, with loss of Max binding correlating with a loss in the biological activity of Myc. A specific viral mutation, isoleucine383>leucine (I383>L) in helix 2 of the HLH domain, extends the LZ domain from four to five heptad repeats. Significantly, introduction of I383>L into a Myc mutant that is defective for Max binding substantially restored its ability to complex with Max in vitro and in vivo. We therefore propose that this virally derived mutation is functional by significantly contributing to establishing a more hydrophobic interface between the LZs of Myc and Max.