Polarization of the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel, to the apical plasma membrane of epithelial cells is critical for vectorial transport of chloride in a variety of epithelia, including the airway, pancreas, intestine, and kidney. However, the motifs that localize CFTR to the apical membrane are unknown. We report that the last 3 amino acids in the COOH-terminus of CFTR (T-R-L) comprise a PDZ-interacting domain that is required for the polarization of CFTR to the apical plasma membrane in human airway and kidney epithelial cells. In addition, the CFTR mutant, S1455X, which lacks the 26 COOH-terminal amino acids, including the PDZ-interacting domain, is mispolarized to the lateral membrane. We also demonstrate that CFTR binds to ezrin-radixin-moesin–binding phosphoprotein 50 (EBP50), an apical membrane PDZ domain–containing protein. We propose that COOH-terminal deletions of CFTR, which represent about 10% of CFTR mutations, result in defective vectorial chloride transport, partly by altering the polarized distribution of CFTR in epithelial cells. Moreover, our data demonstrate that PDZ-interacting domains and PDZ domain–containing proteins play a key role in the apical polarization of ion channels in epithelial cells.
Bryan D. Moyer, Jerod Denton, Katherine H. Karlson, Donna Reynolds, Shusheng Wang, John E. Mickle, Michal Milewski, Garry R. Cutting, William B. Guggino, Min Li, Bruce A. Stanton
Submitter: Edward J. Weinman, MD | firstname.lastname@example.org
University Maryland, School of Medicine
Published April 14, 2000
This letter is written to call attention to the growing problem regarding the naming, and particularly the re-naming, of previously identified proteins. To my knowledge, there are no defined rules. For proteins with a single function, receptors and transporters for example, the name usually reflects its function: i.e. beta adrenergic receptors, sodium-hydrogen exchange transporters. As other isoforms are identified, they are given sequential number designations; NHE1, NHE2 etc. When homologues are isolated, the species designation is indicated by a lower case prefix; hNHE1 for human Sodium-Hydrogen Exchanger Transporter 1. When the protein is multifunctional, however, maintaining the relation between the name and function becomes difficult. This becomes especially problematic when additional functions for the protein are defined. By way of example, the following summarizes this issue as it related to a protein called the Sodium-Hydrogen Exchanger Regulatory Factor; a protein highlighted in recent Perspective articles in the Journal of Clinical Investigation (J. Clin. Invest. 103:761-765;1999, J. Clin. Invest. 103:767 -772;1999).
In the study of cAMP dependent protein kinase regulation of NHE3, it was demonstrated that an additional protein co-factor was required. This protein was subsequently isolated, cloned, and since it had not previously been described, named the Sodium-Hydrogen Exchanger Regulatory Factor; the name reflecting its only known identified function at the time (J. Clin. Invest. 92:1781-1786;1993, J. Clin. Invest. 95:2143- 2149;1995). The initial abbreviations was NHE-RF. Later, the hyphen was dropped. Unfortunately, both the hyphenated and the unhyphenated terms have appeared as Index Terms and Key Words. Approximately 2 years later, NHERF was found to bind ezrin, a very important observation in understanding the physiology of NHERF (Cell Biol. 139:169-179;1997). Without comment, the authors re-named the protein Ezrin Binding Protein 50 (EBP50) although it was identified as the human homologue of NHERF. At the same time, another group identified NHERF binding to merlin, a protein related to ezrin (J. Biol. Chem. 27:1273-1276;1998). This protein was designated hNHERF conforming to the use of a prefix to indicate that they had isolated the human homologue of the rabbit protein. We too contributed to the confusion when a protein similar to NHERF was isolated and named NHE3 Protein Kinase A Regulatory Factor (E3KARP) (Proc. Natl. Acad. Sci. USA. 94:3010- 3015;1997). This protein, like NHERF, supports cAMP relation of NHE3. At the time E3KARP was cloned, an identical sequence was already in GenBank as Tyrosine Kinase Activator 1 (TKA1), although no peer reviewed paper had been written detailing the function of TKA1. Another designation for E3KARP was that it was a NHERF-like protein or was part of the NHERF family of proteins. More recently, the designation NHERF2 has been used.
For those working in the area, the distinction between NHERF and EBP50 may not a major problem but to those trying to understand developments in this field, confusion is great. It was and still is the belief of some fellows and investigators that NHERF and EBP50 represented different proteins. If NHERF were to renamed for each of its functions, the situation would be unmanageable as NHERF has now been demonstrated to interact with a large array of other proteins including receptors, transporters, channels, and other signaling proteins. It might be predicted that NHERF2 also will be found to be associated with additional physiologic processes. At the present time, there seems to be no uniformity in the designation of names. In two recent Perspective articles in the same issue of the Journal of Clinical Investigation, NHERF/EBP50 was used in one while, NHERF was used in the other (J. Clin. Invest. 103:761-765;1999, J. Clin. Invest. 103:767-772;1999). Two recent papers in the Journal Clinical Investigation have examined other functions of NHERF. The first showed an interaction with the Sodium-Bicarbonate Co-Transporter and the name used was NHERF (J. Clin. Invest. 104:195-201;1999). A second paper demonstrated an interaction with Cystic Fibrosis Transmembrane Regulator and the name EBP50 was used (J. Clin. Invest. 104:1353- 1361;1999). The latter paper was authored by investigators that have published previously in the area of regulatory proteins and had used NHERF as the name (FEBS Lett. 427:103-108;1998). In the recent publication in the Journal of Clinical Investigation, there is no indication as to their rationale for the change from NHERF to EBP50. Several informal and unsuccessful attempts have been made to agree on a uniform naming policy. An e-mail was circulated to investigators studying this protein family but no consensus was reached and many did not respond. In a recent publication, Hall et al suggested use of NHERF to preserve the original name and NHERF2 to designate the very similar E3KARP/TKA1 protein (Proc. Natl. Acad. Sci. USA. 95:8496-8501;1998). This has not been adapted uniformly.
It is, perhaps, understandable that a group of investigators would want unique credit for their efforts in isolating individual proteins. If the protein has already been named, however, the re-naming serves to confuse rather than edify. It is sometimes rationalized that the new name represents a more general functional description. This is not always the case and in the example under discussion, additional functions of NHERF are being described that are unrelated to either NHE3 or ezrin. Moreover, there are multiple other examples where the original and retained name of a protein does not reflect its most important or most recently described function. Casual discussion with other investigators from different fields would suggest that the practice of re-naming of known proteins is a common practice. A major journal recently wrote an editorial describing its new policy regarding the naming of proteins, the editorial being precipitated by a publication in its pages about a function of NHERF/EBP50. Since it appears that the issue of naming goes beyond individual proteins and individual investigators, it is suggested that the major scientific journals develop a uniform editorial policy.