A Spin-Tip Enrichment Strategy for Simultaneous Analysis of N-Glycopeptides and Phosphopeptides from Human Pancreatic Tissues

Summary

Post-translational modifications (PTMs) change protein structures and functions. Methods for the simultaneous enrichment of multiple PTM types can maximize coverage in analyses. We present a protocol using dual-functional Ti(IV)-immobilized metal affinity chromatography followed by mass spectrometry for the simultaneous enrichment and analysis of protein N-glycosylation and phosphorylation in pancreatic tissues.

Abstract

Mass spectrometry can provide deep coverage of post-translational modifications (PTMs), although enrichment of these modifications from complex biological matrices is often necessary due to their low stoichiometry in comparison to non-modified analytes. Most enrichment workflows of PTMs on peptides in bottom-up proteomics workflows, where proteins are enzymatically digested before the resulting peptides are analyzed, only enrich one type of modification. It is the entire complement of PTMs, however, that leads to biological functions, and enrichment of a single type of PTM may miss such crosstalk of PTMs. PTM crosstalk has been observed between protein glycosylation and phosphorylation, the two most common PTMs in human proteins and also the two most studied PTMs using mass spectrometry workflows. Using the simultaneous enrichment strategy described herein, both PTMs are enriched from post-mortem human pancreatic tissue, a complex biological matrix. Dual-functional Ti(IV)-immobilized metal affinity chromatography is used to separate various forms of glycosylation and phosphorylation simultaneously in multiple fractions in a convenient spin tip-based method, allowing downstream analyses of potential PTM crosstalk interactions. This enrichment workflow for glyco- and phosphopeptides can be applied to various sample types to achieve deep profiling of multiple PTMs and identify potential target molecules for future studies.

Introduction

Protein post-translational modifications (PTMs) play a major role in modulating protein structures and consequently their functions and downstream biological processes. The diversity of the human proteome increases exponentially due to the combinatorial variability afforded by various PTMs. Different variants of proteins from their canonical sequences as predicted by the genome are known as proteoforms, and many proteoforms arise from PTMs¹. Studying proteoform diversity in health and disease has become an area of research of great interest in recent years^2,3.

Protocol

Consent was obtained for the use of pancreatic tissues for research from the deceased's next of kin and an authorization by the University of Wisconsin-Madison Health Sciences Institutional Review Board was obtained. IRB oversight is not required because it does not involve human subjects as recognized by 45 CFR 46.102(f).

CAUTION: Care should be taken when handling the reagents used in this protocol, which include acids (formic, acetic, trifluoroacetic), bases (ammonium hydroxide), and cr.......

Discussion

The dual-functional Ti(IV)-IMAC strategy is useful for the simultaneous analysis of N-glycopeptides and phosphopeptides from the same sample in a single sample preparation workflow. ERLIC-based methods have also been shown to perform simultaneous enrichment of PTMs. Both strategies have been used previously for deep coverage in PTM analyses^14,18. In adapting the dual Ti method to decreasing sample incubation time by using spin-tips, we hope that this protocol has.......

Materials

Name	Company	Catalog Number	Comments
Acetic Acid, Glacial (Certified ACS)	Fisher Scientific	A38S-500
Acetone (Certified ACS)	Fisher Scientific	A18-1
Acetonitrile, Optima LC/MS Grade	Fisher Scientific	A955-4
Ammonium Acetate (Crystalline/Certified ACS)	Fisher Scientific	A637-500
Ammonium Hydroxide (Certified ACS Plus)	Fisher Scientific	A669-212
Byonic software	Protein Metrics	n/a	Commercial software used for glycoproteomic analysis (https://proteinmetrics.com/byos/)
C18 BEH material	Waters	186002353	Material removed from column and used to pack nano capillaries (pulledto integrate tip used directly in line with instrument inlet)
CAE-Ti-IMAC, 100%	J&K Scientific	2749380-1G	Material used for dual-functional Ti(IV)-IMAC; can also be used for conventional IMAC/conventional phosphopeptide enrichment
Cellcrusher kit	Cellcrusher	n/a	Used for grinding tissue samples into powder before extraction
Eppendorf 5424R Microcentrifuge	Fisher Scientific	05-401-205	For temperature-controlled centrifugation
cOmplete protease inhibitor cocktail tablets	Sigma	11697498001
DTT, Molecular Grade (DL-Dithiothreitol)	Promega	V3151	Protein reducing agent
Ethanol, 200 proof (100%), USP	Fisher	22-032-601
Fisherbrand Analog Vortex Mixer	Fisher Scientific	02-215-414
Fisherbrand Low-Retention Microcentrifuge Tubes (1.5 mL)	Fisher Scientific	02-681-320
Fisherbrand Low-Retention Microcentrifuge Tubes (2 mL)	Fisher Scientific	02-681-321
Fisherbrand Model 120 Sonic Dismembrator	Fisher Scientific	FB120110	For sample lysis using ultrasonication
Formic Acid, 99.0+%, Optima LC/MS Grade	Fisher Scientific	A117-50
Fused silica capillary (75 μm inner diameter, 360 μm outer diameter)	Polymicro Technologies LLC	100 m TSP075375	For in-house pulled and packed columns with integrated emitter
Hydrofluoric acid (48 wt. % in H2O)	Sigma-Aldrich	339261-100ML	Used for opening emitter of pulled capillary column
Iodoacetamide, BioUltra	Sigma	I1149-5G	Protein reducing reagent
MaxQuant software	n/a	n/a	Free software used for phosphoproteomic analysis (https://www.maxquant.org/)
Multi-therm Shaker with heating and cooling	Benchmark Scientific	H5000-HC	Heating block
Oasis HLB 1 cc Vac Cartridge, 10 mg Sorbent per Cartridge, 30 µm, 100/pk	Waters	186000383	Larger-scale cartridge desalting for tryptic digests (loading capacity approximately up to 1 mg each)
OMIX C18 pipette tips, 100 µL tip, 10 - 100 μL elution volume, 1 x 96 tips	Agilent	A57003100	Smaller-scale packed pipette tip for desalting for enrichment elutions
P-2000 Micropipette Puller	Sutter Instrument Co.	P-2000/F	For pulling nano-capillary columns for LC-MS
PhosSTOP phosphatase inhibitor tablets	Sigma	4906845001
Pierce BCA Protein Assay Kit	Thermo Fisher Scientific	23225
Pierce Quantitative Colorimetric Peptide Assay	Thermo Fisher Scientific	23275
PolySAX LP (12 μm, pore size 300 Å)	PolyLC	BMSX1203	Material for strong anion-exchange chromatography used for ERLIC/conventional glycopeptide enrichment
Potassium Phosphate Monobasic (Crystalline/Certified ACS)	Fisher Scientific	P285-500
Pressure injection cell with integrated magnetic stirplate	Next Advance	PC77-MAG	For packing nano-capillary columns with stationary phase up to 2500 psi limit
Proteome Discoverer software	Thermo Fisher Scientific	n/a	Commercial software for proteomics anaysis (with integrated database searching software nodes) and data visualization (https://www.thermofisher.com/us/en/home/industrial/mass-spectrometry/liquid-chromatography-mass-spectrometry-lc-ms/lc-ms-software/multi-omics-data-analysis/proteome-discoverer-software.html)
SpeedVac SC110 Vacuum Concentrator Model SC110-120	Savant	n/a	Centrifugal vacuum concentrator for drying samples (under heat)
SDS Solution, 10% Sodium Dodecyl Sulfate Solution, Molecular Biology/Electrophoresis	Fisher Scientific	BP2436200
Sequencing Grade Modified Trypsin	Promega	V5111
Sodium Chloride (Crystalline/Certified ACS)	Fisher Scientific	S271-500
TopTip, Empty, 10-200 µL, Pack of 96	Glygen Corporation	TT2EMT.96	Empty pipette tip with micron-sized hole used that can be used to pack chromatographic materials for enrichments, bundled with tube adapters
Triethylammonium bicarbonate buffer (TEAB, 1 M, pH 8.5 (volatile))	Sigma	90360-100ML
Trifluoroacetic acid, Reagent Grade, 99%	Fisher Scientific	60-017-61
Tris Base (White Crystals or Crystalline Powder/Molecular Biology)	Fisher Scientific	BP152-500
Trypsin/Lys-C Mix, Mass Spec Grade	Promega	V5071
Urea (Certified ACS)	Fisher Scientific	U15-500
Water, Optima LC/MS Grade	Fisher Scientific	W64