Supercharged proteins are a recently identified class of proteins that have the ability to efficiently deliver functional macromolecules into mammalian cells. They were first developed as bioengineering products, but were later found in the human proteome. In this work, we show that this class of proteins with unusually high net positive charge is frequently found among viral structural proteins, more specifically among capsid proteins. In particular, the capsid proteins of viruses from the Flaviviridae family have all a very high net charge to molecular weight ratio (> +1.07/kDa), thus qualifying as supercharged proteins. This ubiquity raises the hypothesis that supercharged viral capsid proteins may have biological roles that arise from an intrinsic ability to penetrate cells. Dengue virus capsid protein was selected for a detailed experimental analysis. We showed that this protein is able to deliver functional nucleic acids into mammalian cells. The same result was obtained with two isolated domains of this protein, one of them being able to translocate lipid bilayers independently of endocytic routes. Nucleic acids such as siRNA and plasmids were delivered fully functional into cells. The results raise the possibility that the ability to penetrate cells is part of the native biological functions of some viral capsid proteins.

This work was supported by Fundaçao para a Ciência e Tecnologia – Ministério da Educaçaoe Cincia (FCT-MEC, Portugal) [PTDC/QUI-BIQ/112929/ 2009], by the European Union [projects FP7-PEOPLE IRSES (MEMPEPACROSS) and FP7-HEALTH-F3-2008-223414 (LEISHDRUG)], by the Spanish Ministry of Economy and Competitiveness (SAF2011-24899), the Generalitat de Catalunya (2009 SGR 492), by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundaçao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ), and the National Institute of Science andTechnology in Dengue (INCT-Dengue). JMF also acknowledges FCT-MEC for Ph.D. fellowship SFRH/BD/70423/2010. WK is a fellow in the Juan de la Cierva Program of the Spanish Ministry of Science and Innovation