Biology Reference
In-Depth Information
ILD
Key
MAD
Immunoglobulin-like domain
Membrane-anchoring domain
Peptide-binding domain
ILD
MAD
PBD
ILD
MAD
Ancestral Ig-family gene
PBD
PBD
ILD
MAD
Ancestral class II
A
Duplication
PBD
ILD
MAD
PBD
ILD
MAD
Ancestral class II
A
Ancestral class II
B
Duplication
PBD
ILD
MAD
PBD
ILD
MAD
PBD
ILD
MAD
PBD
ILD
MAD
Ancestral class II
A
Ancestral class II
B
Ancestral class II
A
Ancestral class II
B
Deletion
PBD
PBD
ILD
MAD
PBD
ILD
MAD
PBD
ILD
MAD
Class I
A
Class II
A
Class II
B
+
ILD
Class I
B
(
B2m
)
Figure 4.20.
Postulated origin of MHC class I and class II genes (redrawn from Klein
and O'hUigen, 1993). An exon encoding a soluble immunoglobulin-like domain (ILD) is
joined with an exon encoding a membrane-anchoring domain (MAD) to produce an
ancestral immunoglobulin-family gene. This is joined by an exon encoding a peptide-
binding domain (PBD) to produce an ancestral class II-like gene, assumed to be of the A
variety (coding for the α -chain). Duplication and deletion events, along with sequence
divergence would produce the present day array of class I A and class II A and B genes.
The class I B gene is in fact the β 2-microglobin (
B2M
; 15q21-q22) gene, which is not
linked to the main MHC complex.
