Cellular Signaling and G-Proteins

• Plant cells interpret signals and activate correct responses

Principles of Signal Perception & Transduction II - G-proteins & Ion Channels

• G-proteins are essential for signal transduction
• GEFs and GAPs are involved in G-Protein
• Small GTPases plays a key role in signal transduction
• Heterotrimeric Complexes are also important in signal transduction
• Ion channels are important for signal transduction
• Calcium signaling helps signal transduction

Types of G-Proteins

• Molecular switches use GTP/GDP as on/off indicators
• Many are GTPases, which hydrolyze GTP into GDP directly.

Small GTPase Proteins

• Does not have specific trimeric complexes
• Plays essential roles in cellular transport

Heterotrimeric

• Makes 3-part complexes which split apart to transduce a signal from the membrane

G-protein Helpers

• Two kinds of proteins help regulate G-protein activity
• Guanine nucleotide exchange factors (GEFs) aids the exchange of GDP for GTP and turns a G-protein ON
• GTPase-activating proteins (GAPs) aids in the hydrolysis of GTP into GDP and turns a G-protein OFF

G-proteins: Small GTPases

• Critical in Cellular Clipper Card

Function of G-proteins

• "On" will bind to targets, often guiding transport.
• Regulated by inhibitors to keep them "Off".
• Can be membrane bound or in cytosol.
• Small GTPases requires GEFs & GAPs to turn them "On" & "Off".

G-proteins: Heterotrimeric Complexes - Overview

• Make 3-part complexes which split apart to transduce signals from the membrane

G-protein components

• Ga, Gβ, & Gy subunits
• Regulator of G-Signaling (RGS)
• A GTPase-Activating Protein (GAP)
• Turns Ga "Off"
• When “On” Gα has specific downstream targets and the complex breaks apart
• Gẞ & Gy dimer has specific downstream targets

G-proteins: Heterotrimeric Complexes - Ensuring Specificity

• Ensures that there is signal transduction coming from the membrane

Different subunit versions

• Makes different trimeric complex combos
• May only interact with certain signals
• Each complex have different downstream targets
• Transcription factors
• Second Messengers (membrane lipids, ion channels, etc.)

G-proteins: Heterotrimeric Complexes - Plant Bio Edition

• Make 3-part complexes which split apart to transduce signals from the membrane
• Plants do things a bit differently
• Plants has Slightly different relationship to membrane receptors
• Plant unique Extra-Large G-proteins (XLGs)
• Ga subunits are self-activating and GEF is not required
• Negative regulation is rate-limiting, A GAP is required to turn off Ga

Plant Bio Heterotrimeric Complexes

• Ga & XLGs can interact with RLKs & be activated by phosphorylation.

• Extra-Large G-proteins (XLGs) are unique to plants

• Has twice the size of Gα subunit

• Includes Ga domain, Nuclear Localization Signal

• Plant have can operate independent of GTPase activity

• It overlaps with Ga, but also has distinct pathways

• Much greater diversity than plant Ga subunits

• Ga subunits are self-activating and GEF not required

• Leaky" signal when not bound to RGS

• The plant bio edition has a Negative regulation that is rate-limiting

• A GAP is required to turn off Ga

• Can be removed at signal recognition

• Signaling can result in pools or pulses of a signal that grow in intensity when activated & negative regulation is suppressed.

Plant Bio Heterotrimeric Complexes

• Enables measuring of signals
• This could be in regards to Sugar Reserves which can be Level 7
• Water Availability will be Level 2
• Nitrogen Supply is Level 5
• Salt stress is Level 8
• Biotic Stress will be Level 5

G-proteins: Heterotrimeric Complexes - Summary

• Make 3-part complexes which split apart to transduce signals from the membrane
• Overview
• Ensuring Specificity
• RLK Interaction & phosphorylation as activation

G-proteins: Example Pathway & Connection to lon Channels

• Activated Ga subunit & its target convert membrane lipid PIP2 into IP3
• IP3 binds to & activates a calcium channel
• Calcium flows down its gradient & is sensed by a sensor protein (Calmodulin)
• Second Messengers (IP3 & Ca2+)
• Pathways Connecting

Ion flux

• A rapid way to amplify a signal or elicit a specific response.
• Sensor proteins can detect ions or voltage changes.
• Results in Rapid changes in ion concentration that can create other physical responses.
• Potassium ions leaving guard cells
• Example: K+ drawing out water from guard cells by changing water potential following calcium signalling
• Calcium ions entering guard cells help start the process
• Water leaves guard cells by osmosis due to increased water potential in guard cells
• The action potential causes the Guard Cells to become flaccid
• The process works due to a abscisic acid binding to ABA receptors

Ion Channels

• Calcium is a critical Second Messenger

• Minor changes in Ca2+ oscillation patterns communicate specific details on top of if CA2+ is present!

• Calmodulin is a primary Sensor protein

• There are different levels of Ca in vacoules

• Signal Networks are complex systems, with many possible data points & logic-gates

• Specificity/Diversity/Crosstalk

• Intensity & Spatio-Temporal Context

• Networks use RLKs (Ligand x Scaffold x 2nd messengers)

• G-proteins (Subunit Combo x 2nd messengers)

• Iion channels (Type + Calcium oscillations)

• Sesnor protein

Complexity helps systems to be informative

• Specificity/Diversity/Crosstalk
• Intensity & Spatio-Temporal Context
• Indicates where change in a pathway could lead to different signal perception & response

Explore cellular signaling mechanisms, including the role of Calmodulin and ion flux. Understand how calcium oscillation patterns and G-proteins contribute to signal transduction. Learn about GEFs, GAPs, and the products of G-protein activation.