4.15
Gap junctions are specialized membrane channels between neighboring animal cells. They support intercellular signaling by allowing the exchange of ions, second messengers, sugars, and other small molecules between cells.
These channels are made of transmembrane proteins called connexins, or CX proteins.
Six connexin molecules assemble into a hemichannel, also called a connexon. A connexon makes up one half of a complete gap junction channel.
Connexons can be classified into two types: homomeric and heteromeric. When all six connexins are the same type, the structure is called a homomeric connexon.
For instance, in the heart, CX40 can form homomeric connexons. However, CX40 can also combine with another heart connexin, CX43, to form a heteromeric connexon. This heteromeric connexon may have different functions, such as selectivity for specific molecules.
Connexins are first made in the rough endoplasmic reticulum. Many connexins then move to the Golgi apparatus, where they assemble into connexons. The cell then transports the connexons to the plasma membrane. There, they pair with connexons from neighboring cells.
Together, the two hemichannels form a complete gap junction channel that directly connects the cytoplasm of both cells.
Many of these channels cluster together to form structures called gap junction plaques.
In many tissues, gap junction channels remain open under normal conditions, allowing continuous communication between cells.
In the heart, this open state is essential. Gap junctions allow electrical signals to spread rapidly from one cell to the next. Because ions move directly between neighboring cells, large groups of cardiomyocytes contract together in a coordinated, rhythmic pattern.
However, gap junctions are not permanently open. For example, when intracellular calcium levels rise, conformational changes in the connexin proteins cause the channel to close.
多细胞生物利用多种方式使细胞相互通信。间隙连接是特殊的蛋白质,在动物的相邻细胞之间形成孔隙,连接两者之间的细胞质,并允许分子和离子的交换。它们存在于广泛的无脊椎动物和脊椎动物物种中,介导许多功能,包括细胞分化和发育,并且与许多人类疾病相关,包括心脏病和皮肤病。
脊椎动物缝隙连接由跨膜蛋白组成,称为连接蛋白(CX),六个连接蛋白形成一个称为连接子的半环。人类至少有21种不同形式的连接蛋白,几乎在所有的细胞类型中都有表达。当所有六个连接蛋白都是相同的时,连接子半环被称为同型异构体,当不同类型的连接蛋白时,连接子是半同型的。
大多数细胞表达多种类型的连接蛋白。它们可以通过与相邻细胞上的对应物配对,形成功能性连接子半通道或全间隙连接通道。当每个连接子相同时,缝隙连接被认为是同型的,而当它们不同时,缝隙连接被认为是异型的。称为缝隙连接斑块的簇群通常形成于这些通道在斑块中心不断循环和降解中,并在边缘被替换。
缝隙连接允许离子、第二信使、糖和其它小分子在细胞之间通过。这种交换是选择性渗透的,并由通道的连接蛋白组成决定。在某些条件下,它们具有在打开和关闭状态之间切换的能力,允许细胞调节它们之间分子的交换。pH值和Ca2+离子的存在可调节细胞在较短时间尺度上的通讯,而差异基因表达控制发育和成人组织中各种细胞类型连接蛋白的类型和丰度。
Gap junctions are specialized membrane channels between neighboring animal cells. They support intercellular signaling by allowing the exchange of ions, second messengers, sugars, and other small molecules between cells.
These channels are made of transmembrane proteins called connexins, or CX proteins.
Six connexin molecules assemble into a hemichannel, also called a connexon. A connexon makes up one half of a complete gap junction channel.
Connexons can be classified into two types: homomeric and heteromeric. When all six connexins are the same type, the structure is called a homomeric connexon.
For instance, in the heart, CX40 can form homomeric connexons. However, CX40 can also combine with another heart connexin, CX43, to form a heteromeric connexon. This heteromeric connexon may have different functions, such as selectivity for specific molecules.
Connexins are first made in the rough endoplasmic reticulum. Many connexins then move to the Golgi apparatus, where they assemble into connexons. The cell then transports the connexons to the plasma membrane. There, they pair with connexons from neighboring cells.
Together, the two hemichannels form a complete gap junction channel that directly connects the cytoplasm of both cells.
Many of these channels cluster together to form structures called gap junction plaques.
In many tissues, gap junction channels remain open under normal conditions, allowing continuous communication between cells.
In the heart, this open state is essential. Gap junctions allow electrical signals to spread rapidly from one cell to the next. Because ions move directly between neighboring cells, large groups of cardiomyocytes contract together in a coordinated, rhythmic pattern.
However, gap junctions are not permanently open. For example, when intracellular calcium levels rise, conformational changes in the connexin proteins cause the channel to close.
From Chapter 4:
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