The Ad5 fiber mediates nonviral gene transfer in the absence of the whole virus, utilizing a novel cell entry pathway.

The interesting discovery reported here that soluble adenovirus serotype 5 (Ad5) fiber proteins enter cells without the virus was a serendipitous result during our development of Ad5 capsid proteins as nonviral gene transfer vectors. The Ad5 capsid fiber and penton proteins mediate infection. The fiber docks to a noninternalizing cell surface protein called the coxsackievirus-Ad receptor (CAR), followed by penton binding to integrins, triggering integrin-mediated endocytosis of the virus. In our previous work, we assembled the nonviral complex, 3PO, which utilized the penton to mediate gene transfer through integrin binding and endocytosis. Here, we tested whether incorporating the fiber targets 3PO to CAR, thus recapitulating the Ad5 infection pathway. As CAR is not an endocytic receptor, we were surprised to find that the fiber alone, without the penton, enabled gene transfer by binding CAR, but internalizing through an unknown mechanism. We show here that the fiber distributes to the nucleus and cytoplasm after temperature-independent uptake, whereas the penton accumulates around the nucleus after temperature-dependent uptake. Fiber uptake by HeLa cells is also actin-dependent, requires the fiber tail/shaft region, and is largely inhibited by heparin. This study raises the possibility that alternative pathways may enable both viral and nonviral cell entry.