Griffithsin
Created by Firezer Haregu
The antiviral protein Griffithsin (GRFT, PDB ID: 2GUD), extracted from the red algae Griffithsia, is a protein with large biological application. GRFT contains 121 amino acids and has a molecular weight of 12.77 kD. GRFT has an isoelectric point of 5.39. Griffithsin is a mixed specificity lectin, belonging to the Jacalin Lectin family, which binds mannose, glucose, and N-acetylglucosamine. Perhaps the greatest biological application of GRFT is as a microbicide for Human Immunodeficiency Virus type-1(HIV-1). GRFT’s antiviral properties arise from its ability to bind to glycoproteins on the virus’s coat (gp41, gp120, gp160), thus preventing it from entering and infecting a cell. Not only does GRFT prevent HIV-1 entry into T-cells, it also prevents cell to cell fusion and transmission of the virus1. The GRFT dimer has six similar carbohydrate (e.g. mannose) binding sites which help make it a potent binder of glycoproteins2. Other lectins derived from natural organisms require a concentration 25,000 times greater than that required by GRFT to bind to the same amount of HIV-1. GRFT’s potency as an anti-HIV-1 agent makes it a great choice for the development of a microbicide.
Because most HIV-1 infections are a result of heterosexual intercourse, an effective anti-viral which prevents transmission of HIV-1 during sexual contact is of great clinical interest. Namely, a topical microbicide which can be used by women during sexual intercourse could potentially prevent the infection of millions of people worldwide. In vitro experiments have shown that GRFT effectively prevents HIV-1 infection of T-Cells in vaginal lavage fluid (pH 4-8). It was also observed that GRFT remained stable, potent, and had rapid activity in these specific real-life conditions. All of these findings indicate GRFT would make an effective topical anti-HIV-1 vaginal microbicide upon development3.
Griffithsin has also shown to be a potent inhibitor of the SARS Coronavirus (SARS-CoV), which causes Severe Acute Respiratory Syndrome (SARS). Similar to HIV-1 inhibition, by binding gylcoproteins on the SARS-CoV virus, GRFT effectively inhibits viral entry into cells. In laboratory studies, GRFT has been shown to decrease the death rate of mice infected with SARS-CoV4.
A homolog of Griffithsin is Banana Lectin (BanLec, PDB ID: 2BMY). BanLec, which is also part of the Jacalin Lectin family, is found in bananas. BanLec shows high sequence similarity (E-value: 2 x 10-05) with GRFT. BanLec also shares primary and tertiary structural similarities with GRFT as shown by BLAST and DALI searches. ClustalW search shows that the two proteins show strong sequence similarities in multiple regions, including residues 16-39 of GFTRwith residues 24-47 of BanLec. Also, there is strong similarity in GFTR’s residues 51-69 and BanLec’s residues 64-82. These sequence similarities help explain many of the functional similarities shared between GFTR and BanLec. Similar to GRFT, it binds mannose. Furthermore, BanLec binds to the mannose rich viral coat of HIV-1 and prevents its entry into a cell--thus, making it another option for the development of an HIV-1 microbicide5. There are indeed sequence regions that show little to no match between the proteins. Although these proteins have some similar function and are both part of the Jacalin Lectin family, they come from two different species of organisms. This would help to explain the non-conserved regions of their sequences.
Griffithsin is a unique protein in many ways as compared to other proteins in the Jacalin Lectin family. Some of GRFT’s novelties include its unusual domain-swapped (the two monomers break covalent bonds, interlock with one another, and reform covalent bonds) dimeric structure, its three repeated domains, and its six independent binding sites for monosaccharides6.
Griffithsin molecules form a domain-swapped dimer (monomeric forms are rarely observed), in which two β strands of one molecule complete a β prism consisting of three four-stranded sheets, with an approximate 3-fold axis, of another molecule (β-prism-I) 6. GRFT contains three strictly conserved repeats of the sequence GGSGG. These sequences are found in loops connecting the first and fourth strand of each sheet and are very well ordered. The main chain amide of the last residue of each of these sequences helps form a monosaccharide binding site. It is believed that the strict conservation of this GGSGG sequence may be related to the presence of an unusually high number (3) of monosaccharide binding sites on each molecule of GRFT. The first of the repeated GGSGG sequences in griffithsin (residues 8–12) is located between strands 1 and 2 that are swapped between the two molecules forming the dimer. This structural element interacts closely with the carbohydrate (usually mannose) molecule6.
GRFT contains a β-hairpin, consisting of Gly66 and Asp67. This loop is much shorter than in any other lectins, leaving this part of the structure much more open. Glu56, Phe74, Thr76, and Asn77 are all part of a partially buried region that intersects between sheets 2 and 3 and help maintain the spacing between the two. Furthermore, GRFT contains numerous hydrogen bonds throughout the structure. The side chains of Glu56 and Thr76 make a strong hydrogen bond which helps maintain the β-prism-I structure6.
The GRFT dimer has six similar carbohydrate binding sites which help make it a potent binder of glycoproteins3. The sugar binding sites on each monomer are much closer in proximity than in the lectins that belong to the β-prism-II family. This most likely allows for tighter binding of complex carbohydrates (e.g. mannose, glucose, and N-acetylglucosamine). The three carbohydrate binding sites of griffithsin are formed from the parts of the structure with extensive sequence conservation. Therefore, it is not surprising that the main chain amides of these residues can make very similar interactions with the sugars in all three sites. Asp112 is directly involved in mannose binding6.
In conclusion, GRFT has shown it will be a very useful microbicide for HIV-1 and SARS-CoV upon further development. GRFT’s potency and efficacy as an anti-viral agent, as compared to other members of the Jacalin Lectin family, is directly related to its novel assembly of six closely spaced carbohydrate binding sites on a domain-swapped dimer. It's novelty in this sense has opened the door to it's large biological applicability. Such a protein certainly deserves further research to elucidate all of its qualities.