Current Protein and Peptide Science - Volume 3, Issue 5, 2002

In the past decade major advances have been made towards understanding the mechanisms by which polytopic membrane proteins fold and assemble in cellular membranes. In eukaryotes, this process is mediated by a complex set of machinery in the endoplasmic reticulum (ER) that facilitates translocation of peptide loops across and integration of hydrophobic helices into the lipid bilayer. Studies evaluating the biogenesis of P-glycoprotein (P-gp) have been at the forefront of this rapidly expanding field. They have revealed a fascinating although sometimes confusing picture that has challenged our notions about general mechanisms of polytopic protein assembly and questioned specific predictions about the details and uniqueness of P-gp transmembrane topology. This review will attempt to summarize and consolidate our current knowledge of the sequence of events that gives rise to P-gp topology in the ER compartment and the implications of these events for polytopic protein biogenesis and function.

A variety of human cancers become resistant or are intrinsically resistant to treatment with conventional chemotherapy, a phenomenon called multidrug resistance. This broad-based resistance results in large part, but not solely, from overexpression of members of the ATP-binding cassette (ABC) superfamily of membrane transporters, including Pglycoprotein, various members of the multidrug resistance associated proteins (MRPs), and ABCG2, also known as MXR1, BCRP, and ABCP. When overexpressed in cell lines, ABCG2 has the ability to confer high levels of resistance to anthracyclines, mitoxantrone, bisantrene, and the camptothecins topotecan and SN-38. This review focuses on the discovery, the biochemistry and the normal physiology of human ABCG2, a novel ABC half transporter expressed abundantly in placenta, as well as in liver, intestine and stem cells. ABCG2 may serve a protective function by preventing toxins from entering cells as well as potentially playing a role in regulating stem cell differentiation. We also discuss the involvement of ABCG2 in multidrug resistance in cancer, especially with regard to acute myeloid leukemia. The mechanism by which substrates are recognized by ABCG2 and how the energy of ATP hydrolysis is transduced into transport remain elusive. A complete understanding of the mechanism and biological function of ABCG2 will be important for understanding its normal physiology as well as potentially for the development of novel chemotherapeutic treatment strategies.

P-glycoprotein is considered one of the most important member of the rapidly growing superfamily of integral proteins known as the ATP-binding cassette (ABC) which in human also include several other multidrug resistance membrane proteins (i.e., MRP), the product of the cystic fibrosis gene, the TAP-1 / TAP2 peptide transporters encoded by the major histocompatibility complex genes and the gene encoding for breast cancer resistance protein (BCRP) also known as MXR1 (mitoxantrone resistance protein).Many monoclonal antibodies (MAbs) reacting with distinct P-glycoprotein domains have been isolated and used to study the molecular organization and cellular functions of this ABC protein. MAbs have been used for multidrug resistance (mdr) gene cloning, delineation of the secondary and tertiary structure of P-glycoprotein and molecular analysis of the mechanisms involved in substrate recognition and transport. The immunodetection of the distinct products of the mdr gene family in normal and malignant cells and tissues has greatly contributed to the understanding of the physiological role of P-glycoprotein and its possible involvement in the refractory of tumors to chemotherapy. The present article deals with the immunological methods used for the structure-function studies of the P-glycoprotein. After introducing the basic structural features of this ABC transporter, the antibody based-approach is discussed with aiming to furnishing methodological perspectives for further investigations of the physiological role of P-glycoprotein and the multidrug resistance phenomenon.

Bacteria carry a battery of multidrug transporters, which belong to six families of transporters. Members of at least three families the ATP-Binding Cassette superfamily, the Major Facilitator Superfamily and the Multidrug Endosomal Transporter family have been shown to contribute to multidrug resistance phenotype in eukaryotic cells. This review is focused on comparison of bacterial and eukaryotic transporters that do not have a common evolutionary trait and use different sources of energy to perform the transport. Yet they demonstrate an impressive resemblance. All multidrug transporters are capable of recognizing a broad spectrum of structurally diverse compounds. The accumulated data suggest that structural and mechanistic determinants of such ability are similar among unrelated proteins. Despite the apparent similarity, many features are still unique for different classes of transporters. Intriguingly, some cells appear to simultaneously express transporters belonging to different classes. Depending on mechanistic particularities of transporters such concurrent expression can result in synergistic or non-synergistic effects.

ATP binding cassette (ABC) systems constitute one of the most abundant superfamilies of proteins. They are involved in the transport of a wide variety of substances, but also in many cellular processes and in their regulation. In this paper, we made a comparative analysis of the properties of ABC systems and we provide a phylogenetic and functional classification. This analysis will be helpful to accurately annotate ABC systems discovered during the sequencing of the genome of living organisms and to identify the partners of the ABC ATPases.