Schreurs W J, Harold F M
Department of Molecular and Cellular Biology, National Jewish Center for Immunology and Respiratory Medicine, University of Colorado School of Medicine, Denver 80262.
Proc Natl Acad Sci U S A. 1988 Mar;85(5):1534-8. doi: 10.1073/pnas.85.5.1534.
Growing hyphae of Achlya bisexualis drive an electric current through themselves, such that positive charge flows into the apical region (the anterior 300 micron) and exits distally along the hyphal trunk. They also generate a gradient of extracellular pH, such that the medium surrounding the apex is slightly alkaline whereas that along the hyphal trunk is acid. To explore the genesis of these gradients and their relationship to polarized extension, we examined the effects of changes in the composition of the growth medium. The transcellular electric current was most pronounced in medium rich in amino acids. In leaner medium, containing limited amounts of amino acids or none at all, the current was attenuated or absent. We interpret the results to mean that inward current represents H+/amino acid symport, mediated by porters that are preferentially localized in the apical region. Apical alkalinity may be due to ammonia production. Outward current, and perhaps also the generation of metabolic acid, reflects the distribution of the H+-ATPase, which is excluded from the apex but is abundant along the hyphal trunk. Thanks to the spatial segregation of transport functions, protons characteristically flow into the apical region. However, since hyphae grow apically and at the same rate despite wide variations in current pattern, the flow of electric charge through the hyphae cannot be required to polarize extension or to localize the tip.
双性绵霉生长中的菌丝会驱动电流通过自身，使得正电荷流入顶端区域（前端300微米）并沿菌丝主干向远端流出。它们还会产生细胞外pH梯度，使得顶端周围的培养基呈微碱性，而沿菌丝主干的培养基呈酸性。为了探究这些梯度的成因及其与极性延伸的关系，我们研究了生长培养基成分变化的影响。跨细胞电流在富含氨基酸的培养基中最为明显。在氨基酸含量有限或根本不含氨基酸的较稀薄培养基中，电流减弱或消失。我们将这些结果解释为内向电流代表H⁺/氨基酸同向转运，由优先定位于顶端区域的转运体介导。顶端碱化可能是由于氨的产生。外向电流，或许还有代谢酸的产生，反映了H⁺-ATP酶的分布，该酶在顶端区域不存在，但在菌丝主干上大量存在。由于转运功能的空间分隔，质子通常流入顶端区域。然而，尽管电流模式差异很大，但菌丝仍以相同的速率向顶端生长，因此通过菌丝的电荷流动并非极性延伸或定位顶端所必需。
