Small Emitter Tips for Native Mass Spectrometry of Proteins and Protein Complexes from Nonvolatile Buffers That Mimic the Intracellular Environment

Abstract

Salts are often necessary to maintain the native structures and functions of many proteins and protein complexes, but many buffers adversely affect protein analysis by native mass spectrometry (MS). Here, protein and protein complex ions are formed directly from a 150 mM KCl and 25 mM Tris-HCl buffer at pH 7 that is widely used in protein chemistry to mimic the intracellular environment. The protein charge-state distributions are not resolved from electrospray ionization MS using 1.6 μm diameter emitter tips, resulting in no mass information. In contrast, the charge-state distributions are well-resolved using 0.5 μm tips, from which the masses of proteins and protein complexes can be obtained. Adduction of salt to protein ions decreases with decreasing tip size below ∼1.6 μm but not above this size. This suggests that the mechanism for reducing salt adduction is the formation of small initial droplets with on average fewer than one protein molecule per droplet, which lowers the salt:protein ratio in droplets that contain a protein molecule. This is the first demonstration of native mass spectrometry of protein and protein complex ions formed from a buffer containing physiological ionic strengths of nonvolatile salts that mimics the intracellular environment, and this method does not require sample preparation or addition of reagents to the protein solution before or during mass analysis.

Native Mass Spectrometry from Common Buffers with Salts That Mimic the Extracellular Environment

“…We recently proposed that this desalting phenomenon with small emitter tips occurs when initial droplets are small enough to contain on average fewer than one protein molecule,s uch that small droplets that contain ap rotein molecule have al ower ratio of salt to protein than larger droplets that contain on average multiple protein molecules. [20] Them uch larger salt cluster ions observed with the larger tips than with the smaller tips (for example,F igure 2e vs.2 j) is consistent with this mechanism. Thec harge-state distributions of the protein and protein complexes from all six buffers with the submicrometer emitter tips are nearly identical to those from aqueous ammonium acetate with the larger emitter tips,indicating that any effects of acidification and proton-enrichment of the smaller droplets [18] are negligible.…”

“…[7,8,[13][14][15][16][17] Nanoelectrospray (nano-ESI) emitters with tip diameters of about 1 mmor smaller produce ions that have less sodium ion adduction than do those formed using larger tips. [18][19][20] This method has been demonstrated for solutions containing organic solvents and acid in which proteins are denatured and from aqueous solutions with up to 25 mm NaCl. [18,19] It has been proposed that the droplets from the smaller emitter tips undergo more solvent acidification and less solvent evaporation and fission events than larger droplets.…”

“…[18] Recently,s ubmicrometer tips were used to produce resolved charge-state distributions of proteins and protein complexes from a1 50 mm KCl 25 mm Tr is-HCl pH 7b uffer that mimics the intracellular environment. [20] Theoptimum buffer depends on the application. Specific buffer properties,s uch as the ability to inhibit enzymes or coordinate specific metal ions,a re often taken into consideration when choosing abuffer for aspecific application.…”

Breakage of the Oligomeric CaMKII Hub by the Regulatory Segment of the Kinase

“…The submicron emitters were chosen to help reduce salt adducts on the protein ions and to reduce the chemical noise (Susa et al, 2017). Nanoelectrospray ionization was initiated by applying a voltage of ~1.0 kV to 1.2 kV onto a platinum wire (0.127 mm in diameter, Sigma, St. Louis, MO, USA) that is inserted inside the emitter and in contact with sample solutions.…”

Mass spectrometry-enabled structural biology of membrane proteins

“…More recently it has been shown that using submicrometer nano-ESI capillaries (<0.5 mm compared with conventional emitter sizes of 1-10 lm) enable the detection of well-resolved charge state distributions for native proteins and protein complexes analyzed from highsalt biochemical buffers (e.g. 25 mM Tris-HCl, 150 mM NaCl and phosphate buffered saline) [71,72]. This technology has also been demonstrated to be applicable for the analysis of detergent solubilized MPs by nano-ESI (in 150 mM NaCl, 25 mM Tris-HCl, 1.1% (w/v) n-octyl-b-D-glucoside) [73].…”