Divalent Cation-Induced Aggregation of Chromaffin Granule Membranes

Abstract

Divalent cations induce the aggregation of chromaffin granule ghosts (CG membranes) at millimolar concentrations. Monovalent cations produce the same effect at 100-fold higher concentrations. The kinetics of the dimerization phase were followed by light-scattering changes observed in stopped-flow rapid mixing experiments.

The rate constant for Ca²⁺-induced dimerization (k_app) is 0.86-1.0 × 10⁹ M^-1sec^-1, based on the “molar” vesicle concentration. This value is close to the values predicted by theory for the case of diffusion-controlled reaction (7.02 × 10⁹ M^-1sec^-1), indicating that there is no energy barrier to dimerization. Arrhenius plots between 10° and 42°C support this; the activation energy observed, +4.4 Kcal, is close to the value (4.6-4.8 Kcal) predicted for diffusion control according to theory.

Artificial vesicles prepared from CG lipids were also found to have cation-induced aggregation, but the rates (values of k_app) were less than 1/100 as large as those with native CG membranes. Also, significant differences were found with respect to cation specificity. It is concluded that the slow rates are due to the low probability that the segments of membrane which approach will be matched in polar head group composition and disposition. Thus large numbers of approaches are necessary before matched segments come into aposition.

The salient features of the chromaffin granule membrane aggregation mechanism are as follows: (a) In the absence of cations capable of shielding and binding, the membranes are held apart by electrostatic repulsion of their negatively charged surfaces. (b) The divalent and monovalent cation effects on aggregation are due to their ability to shield these charges, allowing a closer approach of the membrane surfaces. (c) The major determinants of the aggregation rates of CG membranes are proteins which protrude from the (phospholipid) surface of the membrane and serve as points of primary contact. Transmembrane contact between these proteins does not require full neutralization of the surface charge and surface potential arising from the negatively charged phospholipids. (d) After contact between proteins is established, the interaction between membranes can be strengthened through transmembrane hydrogen bonding of phosphatidyl ethanolamine polar head groups, divalent cation-mediated salt bridging, and segregation of phosphatidylcholine out of the region of contact.

Key Words:

• stopped-flow rapid mixing
• light scattering
• kinetics
• diffusion-controlled rate constants
• phospholipids
• lateral phase separation