Connexin/Innexin Channels in Cytoplasmic Organelles. Are There Intracellular Gap Junctions? A Hypothesis!

Abstract

This paper proposes the hypothesis that cytoplasmic organelles directly interact with each other and with gap junctions forming intracellular junctions. This hypothesis originated over four decades ago based on the observation that vesicles lining gap junctions of crayfish giant axons contain electron-opaque particles, similar in size to junctional innexons that often appear to directly interact with junctional innexons; similar particles were seen also in the outer membrane of crayfish mitochondria. Indeed, vertebrate connexins assembled into hexameric connexons are present not only in the membranes of the Golgi apparatus but also in those of the mitochondria and endoplasmic reticulum. It seems possible, therefore, that cytoplasmic organelles may be able to exchange small molecules with each other as well as with organelles of coupled cells via gap junctions.

Keywords: calmodulin; cell-to-cell channels; channel gating; connexin; crayfish giant axons; gap junctions; innexin; liver; mitochondria; stomach.

Figure 1

A. Electron micrograph showing the profile of a gap junction between crayfish lateral giant axons (A). The membranes display a beaded profile created by particles (innexons) that are in register, protrude from both membrane surfaces and bind to each other across the extracellular gap (A). The membranes are coated with 500–800 Å vesicles (A, black arrow) whose membrane also contains particles (A, red arrows), similar to junctional particles that often come in direct contact with junctional particles (A and insets a and b). Occasionally, neighboring vesicles appear to bind to each other via particle–particle interactions (A, inset a). The vesicles may directly communicate with each other and with vesicles lining on the other side of the junction (B). A from [9].

Figure 2

Freeze-fracture images of gap junctions between crayfish lateral giant axons (A and B). The neighboring vesicles contain particles and pits (A and B, red arrows) similar in size to the junctional particle (B). Often, the particles of the vesicles display a central dimple similar in size to that of the junctional particles (B, double-headed red arrow). P, Protoplasmic face; E, Exoplasmic face. A from [10].

Figure 3

Freeze-fracture images of gap junctions between rat liver (A) and stomach (B) epithelial cells showing membranes of cytoplasmic cisterns (C) apparently attached to gap junction membranes. P, Protoplasmic face; E, Exoplasmic face. A from [18]; B from [17].

Figure 4

Thin sections of mitochondria in crayfish lateral giant axons. Note that the cross-sectioned outer membrane shows images of electron-opaque particles (A and B, black arrows) that are similar to gap junction particles in size and spacing (~200 Å; B, inset). Similar electron-opaque particles are seen in tangential sections (C, red arrows). From [28].

Figure 5

Hypothetical CL–CL interaction between Cx32 sequences. If present, the interaction might involve numerous hydrophobic residues and include the CL2′s calmodulin (CaM) binding site, which is close to the “32gap 24” amino acid chain. If this were the case, CaM would not bind to the CL2 domain. Above is the predicted secondary structure of this sequence, performed by SCRATCH Protein Predictor, School of Informatics and Computer Sciences (ICS), University of California, Irvine (UCI).

Figure 6

Hypothetical CL–CL interaction between Cx43 sequences. If present, the interaction might involve several hydrophobic residues and include the CL2′s calmodulin (CaM) binding site. Based on the prediction of the secondary structure (see above), the potential CL–CL interacting sequence (R148–K162) is believed to be in alpha-helical conformation. The secondary structure prediction was performed by SCRATCH Protein Predictor, School of Informatics and Computer Sciences (ICS), University of California, Irvine (UCI).

Figure 7

Hypothetical CL–CL interaction between innexin-1 sequences (in squares). If present, the interaction might involve hydrophobic and charged residues and include part of the CL2′s calmodulin (CaM) binding site. The prediction of the secondary structure shown above was performed by SCRATCH Protein Predictor, School of Informatics and Computer Sciences (ICS), University of California, Irvine (UCI).