The Exon Intron Arrangement of Class I and II Genes Reflects Their Domain Structure

Separate exons encode each region of the class I and II proteins (Figure 7-9). Each of the mouse and human class I genes has a 5' leader exon encoding a short signal peptide

L a1 a2

mRNA

Class I MHC molecule

mRNA

Class I MHC molecule

Mhc Exon Related Other Exons

COOH

ß2 - microglobulin

COOH

ß2 - microglobulin

Class II MHC molecule ß

Class II MHC molecule ß

Map Humans Mhc Genes And Gene Product

COOH COOH

mRNA

COOH COOH

mRNA

L a1

FIGURE 7-9

Schematic diagram of (a) class I and (b) class II MHC genes, mRNA transcripts, and protein molecules. There is correspondence between exons and the domains in the gene products; note that the mRNA transcripts are spliced to remove the intron sequences. Each exon, with the exception of the leader (L) exon, en codes a separate domain of the MHC molecule. The leader peptides are removed in a post-translational reaction before the molecules are expressed on the cell surface. The gene encoding ß2-microglobulin is located on a different chromosome. Tm = transmembrane; C = cytoplasmic.

followed by five or six exons encoding the a chain of the class I molecule (see Figure 7-9a). The signal peptide serves to facilitate insertion of the a chain into the endoplasmic reticu-lum and is removed by proteolytic enzymes in the endoplasmic reticulum after translation is completed. The next three exons encode the extracellular a1, a2, and a 3 domains, and the following downstream exon encodes the transmembrane (Tm) region; finally, one or two 3'-terminal exons encode the cytoplasmic domains (C).

Like class I MHC genes, the class II genes are organized into a series of exons and introns mirroring the domain structure of the a and p chains (see Figure 7-9b). Both the a and the p genes encoding mouse and human class II MHC molecules have a leader exon, an a 1 or p 1 exon, an a2 or p2 exon, a transmembrane exon, and one or more cytoplasmic exons.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment