Date Published: June 20, 2019
Publisher: Public Library of Science
Author(s): Sean Joyce, Adel M. Nour, Vladimir Trajkovic.
Autophagy plays a vital role in tumor therapy and survival of dormant tumor cells. Here we describe a novel function of a protein known as Transmembrane 219 (TM219) as an autophagy activator. TM219 is a small membrane protein expressed in all known human tissues except the thymus. We used biochemical approaches to identify calmodulin and calmodulin dependent protein kinase II as a part of TM219 protein complex. Then, we employed in vitro reconstitution system and fluorescence anisotropy to study the requirements of TM219 to bind calmodulin in vitro. We also used this system to study the effects of a synthetic peptide derived from the sequence of the short cytoplasmic tail of TM219 (SCTT) on calmodulin-TM219 receptor interactions. We conjugated SCTT peptide with a pH Low Insertion peptide (pHLIP) for optimal cellular delivery. We finally tested the effects of SCTT-pHLIP on triple negative human breast cancer cells in three dimension culture. Our data defined a novel function of TM219 protein and an efficient approach to inhibit it.
It is estimated that one in every eight women will develop breast cancer in USA. Among the breast cancer patients, there are 15–20% diagnosed with an invasive triple negative tumor  have the worst therapy outcome and have the shortest survival.
TM219 receptor has been linked to different human diseases such as asthma and cancer , . However, the mechanism of how this protein functions has been poorly addressed. The presence of the di-arginine motif (RR, RTR) in the cytoplasmic domain of TM219 suggested that this protein is expressed and retained in the ER compartment. The mechanism of di-arginine motif retention and how membrane proteins with such motif exit the ER compartment has been studied intensively in similar membrane proteins [50–51], [65–68]. Consistent with our observation that TM219 is expressed and retained in ER compartment; TM219-eGFP expression and immunostaining data of TM219 indicated that it is mainly localized in the perinuclear region. Moreover, our biochemical approaches to define proteins that interact with TM219 short cytoplasmic tail identified different proteins that are involved in ER retention and exist of similar membrane proteins.