Cellular aspartyl proteases promote the unconventional secretion of biologically active HIV-1 matrix protein p17.

Francesca Caccuri,Federica Campilongo,Cinzia Giagulli,Silvia Schiarea,Simona Fiorentini,Antonella Bugatti,Arnaldo Caruso,Pietro Mazzuca,Alessandro Rossi,Kristen Varney,Wuyuan Lu,Mario Salmona,Stefania Mitola,Maria Luisa Iaria

doi:10.1038/srep38027

Abstract

The human immune deficiency virus type 1 (HIV-1) matrix protein p17 (p17), although devoid of a signal sequence, is released by infected cells and detected in blood and in different organs and tissues even in HIV-1-infected patients undergoing successful combined antiretroviral therapy (cART). Extracellularly, p17 deregulates the function of different cells involved in AIDS pathogenesis. The mechanism of p17 secretion, particularly during HIV-1 latency, still remains to be elucidated. A recent study showed that HIV-1-infected cells can produce Gag without spreading infection in a model of viral latency. Here we show that in Gag-expressing cells, secretion of biologically active p17 takes place at the plasma membrane and occurs following its interaction with phosphatidylinositol-(4,5)-bisphosphate and its subsequent cleavage from the precursor Gag (Pr55Gag) operated by cellular aspartyl proteases. These enzymes operate a more complex Gag polypeptide proteolysis than the HIV-1 protease, thus hypothetically generating slightly truncated or elongated p17s in their C-terminus. A 17 C-terminal residues excised p17 was found to be structurally and functionally identical to the full-length p17 demonstrating that the final C-terminal region of p17 is irrelevant for the protein’s biological activity. These findings offer new opportunities to identify treatment strategies for inhibiting p17 release in the extracellular microenvironment.

Highlights

By p17 binding to CXCR1 and CXCR2, termed IL-8RA and IL-8RB, and p17 was found to mimic some of the biological activities exerted by IL-817–19
Capture of recombinant p17 was performed by the anti-p17 Monoclonal antibodies (mAbs) MBS-15 coated on 96-well plates, whereas detection was obtained by adding biotin-conjugated anti-p17 mAb MBS-34 and peroxidase-labeled streptavidin
The human immunodeficiency virus type 1 (HIV-1) capsid protein was not detected by the anti-p24 mAbs MBS-1231 even when it was added to the capture well at a concentration as high as 16 nM

Summary

Introduction

By p17 binding to CXCR1 and CXCR2, termed IL-8RA and IL-8RB, and p17 was found to mimic some of the biological activities exerted by IL-817–19. Latently infected resting CD4+ T cells were found to transcribe and translate Gag proteins without stimulation while in a latent state[23], supporting the hypothesis that resting CD4+ T cells can synthesize HIV-1 Gag proteins without contributing to the spread of infection All these findings strongly suggest that p17 may be chronically present in the infected microenvironment, even during pharmacological control of viral replication, in the absence of any HIV-1 protease activity. The HIV-1 transactivating regulatory protein Tat has been recently found to be secreted in an unconventional manner after binding to PI(4,5)P230. This prompted us to investigate possible unconventional routes of p17 secretion via PI(4,5)P2 interaction. The release of p17 occurs following its cleavage from Pr55Gag by cellular aspartyl proteases

Methods

Results

Conclusion