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Review
. 2015 Jul;58(1):1-10.
doi: 10.1016/j.ceca.2015.02.006. Epub 2015 Mar 2.

Organellar channels and transporters

Haoxing Xu  1 , Enrico Martinoia  2 , Ildiko Szabo  3
Affiliations
Review

Organellar channels and transporters

Haoxing Xu et al. Cell Calcium. 2015 Jul.

Abstract

Decades of intensive research have led to the discovery of most plasma membrane ion channels and transporters and the characterization of their physiological functions. In contrast, although over 80% of transport processes occur inside the cells, the ion flux mechanisms across intracellular membranes (the endoplasmic reticulum, Golgi apparatus, endosomes, lysosomes, mitochondria, chloroplasts, and vacuoles) are difficult to investigate and remain poorly understood. Recent technical advances in super-resolution microscopy, organellar electrophysiology, organelle-targeted fluorescence imaging, and organelle proteomics have pushed a large step forward in the research of intracellular ion transport. Many new organellar channels are molecularly identified and electrophysiologically characterized. Additionally, molecular identification of many of these ion channels/transporters has made it possible to study their physiological functions by genetic and pharmacological means. For example, organellar channels have been shown to regulate important cellular processes such as programmed cell death and photosynthesis, and are involved in many different pathologies. This special issue (SI) on organellar channels and transporters aims to provide a forum to discuss the recent advances and to define the standard and open questions in this exciting and rapidly developing field. Along this line, a new Gordon Research Conference dedicated to the multidisciplinary study of intracellular membrane transport proteins will be launched this coming summer.

Keywords: Intracellular channels; Ion channels and transporters; Organellar channel targeting; Organelle membranes.

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Figures

Figure 1
Figure 1. Organellar Channels and Transporters
Intracellular organelles include endosomes, phagosomes, autophagosomes, lysosomes, mitochondria, chloroplasts, plant vacuoles, Golgi apparatus, the ER, peroxisomes, and the nucleus. Intracellular channels are shown as oval objects while transporters and pumps are rectangular. Channels/transporters are color-coded, with calcium-permeable proteins in blue, chloride in green, sodium in yellow, and potassium in violet. Proteins allowing the passage of metabolites and/or several different types of ions are depicted in orange. In the nucleus of plants, Castor and Pollux proteins may mediate potassium flux. In the ER, several calcium transport systems are found (Ryanodine Receptor, IP3 receptor, SERCA pump) as well as cation-permeable channels (TRIC, TRP). Functionally active K+ transport systems are the LETM1 K+/H+ antiporter, and potassium channels (KATP, KCa). In the lysosomes, TRPMLs are permeable to Ca2+ and heavy metals; TPCs are Na+-selective channels, but are also permeable to Ca2+; CLCs are Cl- transporters. TRPs, TPCs, and CLCs are also present in the early endosomes. In the plant vacuoles, TPC1 is the putative Ca2+ channels, while CAXs mediate Ca2+ uptake. TPKs are vacuolar K+ channels, while NHXs mediate H+/Na+ or H+/K+ exchange. CLC proteins function as anion transporters. ALMTs may mediate malate transport. In the mitochondria, only the channels mentioned in this SI are shown – for a complete list see [69]. The MCU complex is responsible for the uptake of calcium. The potassium-permeable pathways in the mitochondria include (K(ATP), K(Ca), Kv1.3 channels, and LETM1 K+/H+ antiporter. MPTP is a large, non-specific pore. In chloroplasts, many metabolite transporters have been identified (see SI Ref. [86]). In addition, ClC-type Cl channel, TPK3 K+ channels, and members of the K+/H+ antiporter KEA family have been identified in chloroplasts.
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