title: MARCKS is a natively unfolded protein with an inaccessible actin-binding site: evidence for long-range intramolecular interactions.
abstract: Myristoylated alanine-rich C kinase substrate (MARCKS) is an unfolded protein that contains well characterized actin-binding sites within the phosphorylation site domain (PSD), yet paradoxically, we now find that intact MARCKS does not bind to actin. Intact MARCKS also does not bind as well to calmodulin as does the PSD alone. Myristoylation at the N terminus alters how calmodulin binds to MARCKS, implying that, despite its unfolded state, the distant N terminus influences binding events at the PSD. We show that the free PSD binds with site specificity to MARCKS, suggesting that long-range intramolecular interactions within MARCKS are also possible. Because of the unusual primary sequence of MARCKS with an overall isoelectric point of 4.2 yet a very basic PSD (overall charge of +13), we speculated that ionic interactions between oppositely charged domains of MARCKS were responsible for long-range interactions within MARCKS that sterically influence binding events at the PSD and that explain the observed differences between properties of the PSD and MARCKS. Consistent with this hypothesis, chemical modifications of MARCKS that neutralize negatively charged residues outside of the PSD allow the PSD to bind to actin and increase the affinity of MARCKS for calmodulin. Similarly, both myristoylation of MARCKS and cleavage of MARCKS by calpain are shown to increase the availability of the PSD so as to activate its actin-binding activity. Because abundant evidence supports the conclusion that MARCKS is an important protein in regulating actin dynamics, our data imply that post-translational modifications of MARCKS are necessary and sufficient to regulate actin-binding activity.
authors.affiliation: Department of Medicine, University of Florida College of Medicine, Gainesville, Florida 32610, USA.
authors.forename: Hazel
authors.initials: H
authors.lastname: Tapp
authors.forename: Iman M
authors.initials: IM
authors.lastname: Al-Naggar
authors.forename: Elena G
authors.initials: EG
authors.lastname: Yarmola
authors.forename: Alexis
authors.initials: A
authors.lastname: Harrison
authors.forename: Gerry
authors.initials: G
authors.lastname: Shaw
authors.forename: Arthur S
authors.initials: AS
authors.lastname: Edison
authors.forename: Michael R
authors.initials: MR
authors.lastname: Bubb
mesh_terms: D000199:Actins / Q000737:chemistry; D000818:Animals; D016880:Anisotropy; D001665:Binding Sites; D002147:Calmodulin / Q000737:chemistry; D002154:Calpain / Q000737:chemistry / Q000494:pharmacology; D004247:DNA / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry*; D007477:Ions; D008565:Membrane Proteins / Q000737:chemistry*; D051379:Mice; D018482:Muscle, Skeletal / Q000378:metabolism; D009227:Myristic Acids / Q000378:metabolism; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D011485:Protein Binding; D011489:Protein Denaturation; D017510:Protein Folding; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013997:Time Factors
chemical_list: D000199:Actins; D002147:Calmodulin; D047908:Intracellular Signaling Peptides and Proteins; D007477:Ions; C000619711:Marcks protein, mouse; D008565:Membrane Proteins; D009227:Myristic Acids; D011994:Recombinant Proteins; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D004247:DNA; D002154:Calpain