MAFA

For other uses, see Mafa (disambiguation).

MAFA (Mast cell function-associated antigen) is a type II membrane glycoprotein, first identified on the surface of rat mucosal-type mast cells of the RBL-2H3 line. More recently, human and mouse homologues of MAFA have been discovered yet also (or only) expressed by NK and T-cells.^[1] MAFA is closely linked with the type 1 Fcɛ receptors in not only mucosal mast cells of humans and mice but also in the serosal mast cells of these same organisms.^[2]

It has the ability to function as both a channel for calcium ions along with interact with other receptors to inhibit certain cell processes. It function is based on its specialized structure, which contains many specialized motifs and sequences that allow its functions to take place.^[3]

Discovery

Experimental discovery

MAFA was initially discovered by Enrique Ortega and Israel Pecht in 1988 while studying the type 1 Fcɛ receptors (FcɛRI) and the unknown Ca²⁺ channels that allowed these receptors to work in the cellular membrane. Ortega and Pecht experimented through using a series of monoclonal antibodies on the RBL -2H3 line of rat mast cells. While experimenting and trying to find a specific antibody that would raise a response, the G63 monoclonal antibody was shown to raise a response by inhibiting the cellular secretions linked to the FcɛRI receptors in these rat mucosal mast cells. The G63 antibody attached to a specific membrane receptor protein that caused the inhibition process to occur. Specifically, the inhibition occurred by the G63 antibody and glycoprotein cross-linking so that the processes of inflammation mediator formation, Ca2+ intake into the cell, and the hydrolysis of phosphatidylinositides were all stopped. This caused biochemical inhibition of the normal FcɛRI response. The identified receptor protein was then isolated and studied where it was found that when cross-linked, the protein actually had a conformational change that localized the FcɛRI receptors. Based on these results, both Ortega and Pecht named this newly discovered protein Mast cell function-associated antigen or MAFA for short.^[2]

Structure and coding

Protein structure

General structure

MAFA is said to be a type II membrane glycoprotein, which means that its N-terminus will face the cytosol while its C-terminus will face the extracellular environment. The protein is 188 amino acids in length and has both hydrophobic and hydrophilic regions within these amino acids. The MAFA protein weighs between 28 and 40 kilodaltons and can exist as both a monomer or a homodimer in various species as seen by the SDS-PAGE results that show two broad bands based on these two forms.^[2] The MAFA core polypeptide sequence weights about 19 kilodaltons, however, a large amount of the weight comes from the N-linked oligosaccharides that are attached onto the protein. This heavy glycosylation is a common occurrence among type II membrane glycoproteins and is a key part of both their structure and function. The variation among glycosylation levels helps play an important role in the properties of MAFA proteins, so the protein must be properly made and modified in order to have full functionality.^[4]

CRD region

The C-terminus of MAFA contains 114 amino acids and has a distinct region called the carbohydrate recognition domain, or CRD for short. This region, as implied in the name, is where various carbohydrates and signaling molecules are recognized and attach to the protein. This CRD is present in many other glycoproteins present in higher level eukaryotes. The CRD is distinguished by a conserved 15 amino acid sequence that includes the following number of amino acids: two glycine residues, two leucine residues, five tryptophan residues, and six cysteine residues. These residues help to form various motifs through their interactions including both WIGL and CYYF motifs.^[4]

Intracellular domain

Along with specialized sequences on both the N terminus and C terminus, the intracellular domain of this protein contains a specialized sequence called the SIYSTL sequence, where the name is the one letter amino acid abbreviations of its residues.^[5] All of the amino acids in this sequence are polar in nature and are considered to be a part of the Immunoreceptor Tyrosine-based Inhibitory Motif (ITM). This ITIM allows the MAFA receptor protein to not only be considered a type II glycoprotein, but is also classified as an inhibitory receptor.^[5]

Genetic coding

Transcriptional and translational coding

As with other proteins, the MAFA undergoes both transcription followed by translation and post-translational modifications in the ER and Golgi. The genomic coding region of this protein consists of 13 kilobytes of genetic information with five exons that are split by four introns in the gene. Of these five exons, three are used to help code the CRD region that was previously mentioned. This gene is also regulated through an upstream promoter region that is 664 basepairs up from the first nucleotide of the protein. Like other proteins, the gene is copied in multiple starting points and put together into an mRNA transcript.^[6]

Alternative splicing

After the code was transcribed into mRNA, the MAFA strand was also found to undergo alternative splicing which has allowed various forms of the MAFA protein to be translated and lead to many of the variations previously discussed. One form of this code deletes the transmembrane portion of the MAFA protein and causes a soluble version to be made, being unique to this protein and has allowed scientists to apply this alternative splicing idea to other Mast cell transmembrane proteins as well.^[4] Once translated, the protein enters the proper cellular pathways from the ER to the Golgi and eventually the cellular membrane, where it is integrated and begins its functionality.