Samples were delipidated by adding 1.5 mL of ice cold acetone
and mixing well, put at −80 °C overnight and one tenth fraction removed
for protein quantification. Samples were centrifuged at
7000 rpm for 1 h at 4 °C. Acetone was removed and the pellet dried
down in a nitrogen stream and stored at −80 °C. Protein quantification
of the aliquot was determined using the BCA quantification kit
(Thermo Fisher Scientific 23225) as directed. Lipid droplet protein
fractions were pooled for at least 50 μg protein per sample. The sample
preparation was done in three biological replicates (3 N2 versus 3
daf-2) to evaluate reproducibility and statistical significance of the
differential abundance values.
Given the small sample amount we decided on the trifluoroethanol
based protocol [25] for preparation of the samples for the LC–MS/MS
analysis. Briefly, the sample was resolubilized in 50 μl of NH4HCO3
(pH 7.8) using short pulses in 5510 Branson ultrasonic water bath
(Branson Ultrasonics) for 3 min. After adding 50 μl of trifluoroethanol,
to denature the proteins the samples were incubated at 60 °C for 2 h
with gentle shaking (1000 rpm). Upon incubation the samples were diluted
5-fold with NH4HCO3 (pH 7.8), supplemented with trypsin
(Promega) 1:25 w/w trypsin-to-protein ratio and additionally incubated
for 3 h at 37 °C to digest the proteins into peptides. After the digestion,
the samples were dried up using Speed-Vac and resolubilized in
nanopure water. The 0.5-μg aliquots were analyzed on an LTQ Orbitrap
Velos mass spectrometer that was interfaced with a 75 μm i.d. 65 cm
long LC column packed with 3-μm Jupiter C18 particles (Phenomenex)
as has been described before [26]. The acquiredMS/MS spectra datasets
were preprocessed with DeconMSn [27] and DtaRefinery [28] software
followed by spectra interpretation using MSGFplus [29] software by
matching against WormBase WS210 protein fasta file. The resulting
peptide-to-spectrum matches were processed by MSnID Bioconductor
package to ensure the maximum number of peptide identifications
while not exceeding 1% of false discovery rate. After exporting the results
as a spectral counting cross-tab (rows are proteins, columns are
samples and values in the table are the number ofMS/MS identifications
of peptides belonging to a given protein) we applied Poisson-based
model (using msmsTests Bioconductor package) to discover proteins
differentially abundant between the lipid droplets from N2 and daf-2
mutant strains.
2.4. Fatty acid composition and lipid analysis of isolated lipid