1.A.27 The Phospholemman (PLM) Family

The PLM family includes mammalian phospholemmans of 8-10 kDa size. They span the membrane once with their N-termini outside. These proteins induce a hyperpolarization-activated chloride current in Xenopus oocytes. They are found in muscle and many body tissues and are targets of protein kinases A and C. Other possible members include the chloride-conductance inducer protein, Mat8, and Na⁺/K⁺-ATPase γ-subunit 'proteolipids.' These proteins are smaller, but of the same orientation in the membrane (see below).

PLM forms anion-selective channels when reconstituted in planar lipid bilayers. These channels display a linear current-voltage relationship, have a unitary conductance and are open most of the time at voltages between -70 and +70mV. The PLM channel is permeable to both organic and inorganic anions including chloride, taurine, lactate, glutamate, isethionate, and gluconate. These channel proteins resemble cardiac γ-subunits of the Na⁺,K⁺-ATPase (FXYD) (TC #3.A.3).

FXYD5, a member of the FXYD family of single spanning type I membrane proteins (also called FXYD-containing ion transporter regulator 5) interacts with and regulates ion channels and the Na⁺,K⁺-ATPase (Lubarski et al., 2005). The same is true for at least 5 other FXYD proteins including FXYD1. These proteins are members of the dysadherin superfamily of ion channels and ion channel regulators (Lifshitz et al., 2006). The cytoplasmic tail of PLM interacts with the intracellular loop of the cardiac Na⁺/Ca²⁺ exchanger (Wang et al., 2006).  PLM coordinately regulates the cardiac Na⁺/Ca²⁺ exchanger and the Na⁺-K⁺-ATPase (Cheung et al. 2013).

Many members of this superfamily have been characterized. FXYD5 is of 178 aas and has N-terminal (residues 7-24) and C-terminal (residues 146-162) hydrophobic regions. These proteins display a short region (130-167) with striking sequence similarity (50% identity) to established members of the PLM family. Since the region of sequence similarity includes a transmembrane domain, these auxiliary proteins may have anion-selective channel activity.

The generalized transport reaction catalyzed by PLM and Mat8 is:

Anions (out) Anions (in).