Many peripheral membrane proteins contain unstructured clusters of basic and aromatic residues that interact with lipid bilayers. In some cases, these clusters can bind phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ) 1 (Fig. 1C), a key regulator of signal transduction in cell membranes (for reviews, see Refs. 1-8). For instance, binding of PI(4,5)P 2 to the basic-aromatic cluster on phospholipase D activates the enzyme (9 -11), whereas a similar cluster in the effector domain (residues 151-175) of the myristoylated alanine-rich C kinase substrate (MARCKS) protein may act as a reversible buffer for regulating free PI(4,5)P 2 levels in the plasma membrane (2). Peptides corresponding to basic-aromatic clusters of phospholipase D and MARCKS bind PI(4,5)P 2 with high affinity and specificity compared with monovalent acid lipids such as phosphatidylserine (11-13). To establish how these clusters can bind to the membrane, selectively sequester PI(4,5)P 2 , and possibly exert other effects on membrane structure, we need to establish their specific location within the bilayer. This study focuses on the effector domain of the MARCKS protein, one of the major substrates for protein kinase C (for reviews, see Refs. 14 -16). MARCKS associates with membranes through an N-terminal myristoyl group and a cluster of basic-aromatic amino acids in its effector domain (16). The five Phe residues in the basic effector domain may influence membrane binding or structure in several different ways, depending on the degree to which they penetrate the membrane bilayer. For example, as we discuss in more detail below, they could (i) contribute a hydrophobic term to the membrane-binding energy, (ii) enhance the electrostatic potential due to basic residues in the cluster, or (iii) induce membrane curvature. Determining the depth of the Phe residues within the membrane is the first step in understanding why they are often associated with basic residues in peripheral membrane proteins.An array of biophysical approaches has shed light on the conformation and location of the MARCKS-(151-175) peptide bound to lipid membranes. EPR spectroscopy (17) and circular dichroism (13) have shown that this peptide adopts an extended conformation when bound to membrane bilayers. Cafiso and co-workers (18, 19) have undertaken an extensive series of EPR studies on MARCKS-(151-175) to probe its location relative to the membrane surface. They systematically substituted amino acids in the MARCKS-(151-175) sequence with cysteines derivatized with nitroxide spin labels and found that spin labels attached at the highly basic and hydrophilic N and C termini of the peptide reside on the aqueous side of the lipid phosphate head group, whereas spin labels attached to the central Phe-containing portions of the peptide are located several angstroms below the lipid head group (17).In this study, we used a complementary technique, magic