The ATP binding cassette (ABC) superfamily of membrane transporters is one of the largest protein classes known, and counts for numerous proteins involved in trafficking of biological molecules across membranes, host-defense mechanism to xenobiotics. The first known members were P-glycoprotein (P-gp) and multidrug resistant protein (MRP), cause multidrug resistance when transfected into drug-sensitive cells. In addition, increasing numbers of ABC proteins have recently been identified. The human ABCG1 (ABC, subfamily G, member 1) gene encodes a member of ABC superfamily that mediates the ATP-dependent translocation of variety of amphiphilic and lipophilic molecules. ABCG2 has been identified as a candidate protein responsible for cancer multidrug resistance, the overexpression of ABCG2 was found in several drug-selective cell lines. Search made of EST databases with BLAST program led to identification of several mouse and rat sequences that had high homology to ABCG2 but that appeared to encode a unique gene. ABCG3 is the most closely related to ABCG2 with 54% amino acid identity overall. The gene, ABCG4, produces several transcripts that differ at the 5' end and encode proteins of various lengths, the ABCG4 protein is closely related to the Drosophila's white and human ABCG1 genes, and belongs to the ABCG subfamily which are involved in cholesterol transport. ABCG5 and ABCG8 are members of the G subfamily of ABC transporters, which are predicted to contain a single magnesium-dependent ATP catalytic domain N-terminal to six transmembrane segments, mutations in either of them cause an identical phenotype which is consistent with these two gene products functioning as heterodimer. ABCG6 and ABCG7 exist in Dictyostelium species of eukaryotes.
Since the above proteins were able to transport substances across cellular membranes and against concentration gradient they require an input of energy, which requires the hydrolysis of ATP, directly or indirectly.

ABCG1 a 678aa protein in human (chr 21q22.3), 666aa in mouse, predominantly expressed in many tissues, in the intracellular compartments mainly associated with endoplasmic reticulum and Golgi complex. It shows 26% sequence homology with ABCG2 protein. ABCG1 protein is involved in macrophage lipid homeostasis, an active member of macrophage lipid export complex.

ABCG2 protein has 655aa in human (Chr 4) and 657 in mouse, is an integral membrane protein, which plays a major role in multidrug resistance phenotype of malignant cells, when overexpresssed. It appears in three different forms, containing Arg, Gly or Thr at aa position 482, these variants posses significant differences in their cross-resistance and drug transport patterns. The human ABCG2 protein shows 54% sequence homology with mouse ABCG3 protein.

ABCG3 a 650aa protein in mouse, highly expressed in thymus and spleen. It seems to have defective ATP binding region, which suggest ABCG3 protein may not bind or hydrolyze ATP, therefore have to dimerize with another subunit to form a functional transporter. It is unique among ABC genes in not having several highly conserved residues in the A and C domains of NBF. ABCG3 is the most closely related to ABCG2 with 54% homology overall, 64% in the NBF and 50% in the TM region.

ABCG4 protein is highly expressed in both human and mouse brain, it is a 646aa molecule in human (chr 11q23) and mouse. It is an integral membrane protein may be involved in macrophage lipid homeostasis. The ABCG4 protein's abundant expression in brain and close evolutionary relationship to the other members of the subfamily suggests a potential role in cholesterol transport.

ABCG5 gene product a 651aa residue in human, and 652aa each in rat and mouse, plays an important role in the selective transport of cholesterol in and out of the enterocytes and in the selective sterol excretion by liver into bile. Strongly expressed in liver, intestine and colon. Defects in ABCG5 results increased intestinal absorption of sterols and decreased biliary excretion of dietary sterols into bile; the condition is referred as sitosterolemia.

ABCG8 protein is a 672aa residue in rat and 673aa each in mouse and human. It may form heterodimers with ABCG5 or be tightly coupled to ABCG5 along a pathway regulating dietary-sterol absorption and excretion. ABCG8 protein is highly expressed in liver, intestine and colon. Defects in ABCG8 cause sitosterolemia.

Rb=rabbit; m=mouse; r=rat; h=human; s=sheep; b=bovine; c=chicken; d=dog; ~CT or ~NT=near C or N-terminus. EC=Extracellular; CL=Cytoplasmic loop.

* Expected antibody crossreactivity information is mostly based upon high (>70%) sequence conservation of antigenic/control peptides in various species. When antibody crossreactivity has actually been experimentally confirmed in various species, it will be mentioned in the appropriate data sheets.

"Neat Antisera or antisera" are the unpurified antiserum and it is suitable for ELISA and Western.
"Affinity pure" IgG may be more suitable for immunohistochemical (IHC) applications and to reduce background in most immunological applications including ELISA and Western.
"Control peptides" can not be used for Western as they are very short peptides. They are intended for ELISA or antibody blocking studies to establish antibody specificity.
Western blot +ve protein controls, where available, are semi-pure, pure or recombinant proteins that are formulated in SDS-PAGE sample buffer. They are recommended to be used for Western (load 10 ul/lane) for visualization with antibodies.