The study of viruses necessarily involves dissecting the intimate details of cellular pathways. Viruses have often been employed as tools in studying cellular pathways, as was done by early retrovirologists such as Peyton Rous in attempting to understand the mechanism of cellular transformation and oncogenesis. On the other side of the coin, virologists seek to de?ne those cellular elements interacting intimatelywiththeir virus ofinterestinorder to better understand viral replication itself, and in some cases to develop antiviral strategies. It is in the intersection of virology and cell biology that many of us ?nd the most rewarding aspects of our research. When a new discovery yields insights into basic cellular mechanisms and presents new targets for int- vention to ?ght a serious pathogen, the impact can be high and the excitement intense. HIV has been no exception to the rule that viruses reveal many basic aspects of cellular biology. In recent years, in part because of the importance of HIV as a major cause of human suffering, numerous cellular processes have been elucidated through work on processes or proteins of this human retrovirus. The excitement in this ?eld is especially well illustrated by the discovery of new innate means of resisting viral replication, such as the work on APOBEC3G, TRIM5a, and BST-2/ tetherin presented in this volume.

"The authors seek to understand the assembly process of human immunodeficiency virus (HIV) and to develop a vaccine that can effectively neutralize HIV. HIV assembly is a process directed by the viral Gag polyprotein. Gag is a myristoylated precursor protein that is translated in the cytoplasm and then traffics to the plasma membrane or to endosomal vesicles for assembly. The authors have recently described an interaction between Gag and the delta subunit of the AP-3 adaptor protein complex (1). The mechanism and structural basis for this interaction is now under intense study. Our hypothesis is that the AP-3 interaction is responsible for the trafficking of Gag to the multivesicular body (MVB) and that this event is part of a normal productive particle assembly pathway. The Vpu protein of HIV assists the virus assembly process through a poorly-defined mechanism. The authors have presented evidence that Vpu overcomes a host cell restriction to assembly (2). Vpu appears to act through interactions with the recycling pathways in the cell, rather than directly interacting with Gag (3). The nature of the host restriction to assembly that Vpu overcomes is a focus of our ongoing work."

Host Restriction of HIV-1 by APOBEC3 and Viral Evasion Through Vif.- Interactions of Viral protein U (Vpu) with Cellular Factors.- TRIM5alpha.- Human Immunodeficiency Virus Type-1 Gag and Host Vesicular Trafficking Pathways.- The Roles of Tetraspanins in HIV-1 Replication.- Imaging of HIV/Host Protein Interactions.- Virological and Cellular Roles of the Transcriptional Coactivator LEDGF/p75.- Implications of Nef: Host Cell Interactions in Viral Persistence and Progression to AIDS.- Vpr and Its Interactions with Cellular Proteins.