Yet they do not yet know whether the protein is present in preparations of the plant commonly sold as a dietary supplement for depression.
The researchers did not use the supplement as a source for the plant extracts that they went on to culture in the lab and therefore St John's wort extracts should not be considered as a treatment for patients infected with HIV-1, they said.
However their findings, to be published online in the 27 October issue of Gene Therapy, are an interesting indication of the potential anti-viral properties of the herbal.
Lead author Kamel Khalili, director of the Temple University's department of neuroscience and Center for Neurovirology (CNV), and colleagues from Yerevan State University in Armenia were originally examining St John's wort extracts to see if they had any effect on cell growth or the behaviour of brain cells in vitro.
"During the course of that study, we also looked to see whether these plant extracts that we had isolated from the callus culture had any anti-viral activity," said Khalili. "We soon discovered that the plant extract inhibited HIV-1 gene expression and replication in infected cells."
The team then isolated the protein from the plant extract responsible for the observed anti-viral activity. After identifying the protein, the group cloned the gene, which they realized was a novel protein and named p27SJ.
"It has unique characteristics," said Khalili. "Remember, it is a plant protein, and so far, to my knowledge, there is no similar protein to that in mammalian cells."
After cloning the gene, the researchers then were able to identify the molecular mechanism by which the protein is able to suppress HIV-1 gene expression and replication, according to Khalili.
It is the expression of the viral gene and the replication of the viral genome that leads to the development of AIDS in HIV-infected individuals.
"Our studies indicate that p27SJ has the capacity to inhibit expression of the HIV-1 gene by interacting with both cellular proteins and viral proteins," said Khalili. "Since HIV-1 gene expression relies heavily on these factors, p27SJ can block viral replication by interfering with the proteins recruited by HIV-1 to increase viral gene expression."
But he warned that the researchers do not know yet how to deliver the protein to cells infected with HIV-1.
And even if the protein were present in dietary supplements, "we don't know how much might be present and whether the protein would be effective when ingested", he added.