New insights of how the HIV-1 assembles and incorporates the Env protein

Assembly of HIV-1, which causes AIDS, takes place on the inner plasma membrane leaflet of infected cells, a geometric building process that creates hexamers out of trimers of the viral Gag protein, as guided by Gag’s N-terminal matrix domain.

Yet certain details of that virion assembly have been lacking for four decades. In a study published in the journal Proceedings of the National Academy of Sciences of the United States of America, Jamil Saad, Ph.D., University of Alabama at Birmingham (UAB), and colleagues provide the first atomic view of the matrix lattice, a step that advances the understanding of key mechanisms of viral assembly and viral envelope protein incorporation.

“Our findings may facilitate the development of new therapeutic agents that inhibit HIV-1 assembly, envelope incorporation and ultimately virus production,” said Saad, a professor of microbiology at UAB.

The Gag protein is post-translationally modified, in which a lipid-like myristate group is added to help Gag bind to the plasma membrane. How the myristoylated matrix domain, or myrMA, of Gag assembles into lattice eluded detection until now. 

Techniques with low molecular resolution — such as cryo-electron diffraction and cryo-electron tomography — suggested that the myrMA protein organizes as trimers, and these trimers then undergo higher-order organization to form hexamers of trimers. Saad’s study is consistent with a recent study, which suggested that the myrMA protein undergoes dramatic structural changes to allow the formation of distinct hexameric lattices in immature and mature viral particles. Virus maturation is the last step of the virus replication cycle, as the capsid core forms inside the assembled virus, yielding infectious particles.

The envelope protein of HIV-1, or Env, is a transmembrane protein delivered to the plasma membrane by the cell’s secretory pathway. The bulk of the Env protein extends beyond the membrane, but a tail hangs through the membrane back into the inside of the cell. Genetic and biochemical studies have suggested that incorporation of the viral Env protein into the virus particles also depends on interaction between the myrMA domain and the cytoplasmic tail of Env. In 2017, Saad’s lab solved the high-resolution structure of the cytoplasmic tail of Env, which was the last unknown protein structure of HIV-1.

Env is a key infectivity protein. As a mature HIV-1 virus approaches a target cell, Env attaches to proteins on the outside of the uninfected cell, and then the Env protein snaps like a mousetrap to fuse the viral membrane with the cell membrane. 

The structures described by Saad and UAB colleagues showing molecular details at 2.1- angstroms resolution were determined via x-ray diffraction data collected at the Southeast Regional Collaborative Access Team (SER-CAT) beamline 22-ID at the Advance Photon Source. The structures show that the myristic acid of myrMA plays a key role in stabilizing the lattice structure, so the ability to form crystals of myrMA was important. They achieved this elusive technical challenge by removing 20 amino acids from the end of the 132-amino acid myrMA. Formation of a Gag lattice on the plasma membrane is known to be obligatory for the assembly of immature HIV-1 and Env incorporation. 

Read more on the Argonne website

Image: X-ray crystallography revealed the structure of the HIV-1 matrix protein at 2.1 angstroms resolution, advancing understanding of key mechanisms of viral assembly.

 Understanding how the HIV virus evades immune surveillance

About 36 million people have died from AIDS-related illnesses and approximately 38 million people globally are living with HIV.

Dr. Jonathan Cook, a resident physician specializing in medical microbiology at the University of Toronto, is investigating key proteins on the HIV virus that are crucial to developing an effective vaccine.

“These proteins are so interesting because they are necessary for a virus to infect a human,” said Cook. “By blocking their function, we can avert the kinds of infections that you see routinely.”

He and Adree Khondker in the lab of Prof. Jeffrey E. Lee from the Temerty Faculty of Medicine published a paper in Communications Biology that reveals new information on how the HIV virus interacts with immune systems.

Using the CMFC beamline at the Canadian Light Source at the University of Saskatchewan, the research team analyzed the outer proteins on the HIV virus. They discovered that an area of one protein acts as a decoy — diverting the immune system’s response towards a false target.

This tactic allows the virus to successfully infect human cells and to cause disease.

“The immune system recognizes this sequence on the virus, which is usually a good thing. But, in this situation, the antibodies that the immune system makes don’t protect you from infection,” Cook said.

With the help of the CLS, the researchers confirmed that this decoy area on the HIV protein shapeshifts to entice an ineffective immune response.

Read more on the CLS website

Image: Micrographs of crystals from this project that were diffracted at CLS

Experimental drug targets HIV in a novel way

SCIENTIFIC ACHIEVEMENT

Using the Advanced Light Source (ALS), researchers from Gilead Sciences Inc. solved the structure of an experimental HIV drug bound to a novel target: the capsid protein that forms a shield around the viral RNA.

SIGNIFICANCE AND IMPACT

The work could lead to a long-lasting treatment for HIV that overcomes the problem of drug resistance and avoids the need for burdensome daily pill-taking.

Progress in HIV treatment still needed

Over the past couple of decades, safe and effective treatment for HIV infection has turned what was once a death sentence into a chronic disease. Today, people on the latest HIV drugs have near-normal life expectancy.

However, many people are still living with multidrug-resistant HIV, unable to control their virus loads with currently available HIV drugs. The virus develops resistance when people take their pills inconsistently due to forgetfulness, side effects, or a complex schedule. To some, taking a pill every day is a burden: they schedule their lives around it for fear of missing a dose. To others, it is a stigma, as it makes it difficult to hide one’s HIV status from close friends and family.

Read more on the Advanced Light Source website

Image: An experimental small-molecule drug (GS-6207) targets the protein building blocks of the HIV capsid—a conical shell (colored red in this artistic rendering) that encloses and protects the viral RNA—making the virus unable to replicate in cells. Credit Advanced Light Source

New possibilities against the HIV epidemic

Research identifies new antibodies with potent activity against virus and infected cells

The Human Immunodeficiency Virus type-1 (HIV-1) currently infects 37 million people worldwide, with an additional 2 million new infections each year. Following infection, the virus has a long period of latency, during which it multiplies without causing symptoms. HIV attacks the cells of the immune system, especially the cells called CD4+ T-lymphocytes, which are responsible for triggering the body’s response chain against infections. Thus, by suppressing the action of the immune system, the virus destroys the body’s ability to defend itself against other diseases, leading to the so-called Acquired Immunodeficiency Syndrome, or AIDS.
Even with the development of antiretroviral therapies that have improved quality of life and increased the life expectancy of patients with HIV/AIDS, it is widely accepted that the only way to effectively curb this devastating epidemic is through the development of an HIV-1 vaccine.

>Read more on the Brazilian Synchrotron Light Laboratory website

Image: Part of the structure of the CAP228-16H protein with the region of the V2 loop highlighted in yellow. (Full image here)