Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Figure 1 Schematic model of ATP-binding cassette transporters P-glycoprotein, multidrug resistance-associated protein 1 and breast cancer resistance protein. The functional unit of P-gp consists of two NBDs and two TMDs containing 12 (2 × 6) membrane-spanning alpha helices. MRP1 also has a core structure containing two NBDs and two TMDs. Besides, it still has a third TMD (TMD0) with five predicted transmembrane segments and an extra N-terminus. BCRP is a "half transporter", consisting of only one NBD and one TMD. BCRP: Breast cancer resistance protein; MRP1: Multidrug resistance-associated protein 1; NBD: Nucleotide-binding domain; P-gp: P-glycoprotein; TMD: Transmembrane domain.